26 September 2026

Your Computer Has Users — Even When Only You Use It

If you are the only person who uses your computer, it is tempting to think that the idea of "users" does not really matter.

You switch it on. You log in. You use it.

Surely there is just you.

But look inside almost any modern operating system and you discover something rather different.

Your computer may have many users and groups, even though only one human being normally sits in front of it.

Some belong to real people. Others exist for software, services and administration. Each may be allowed to see, modify or run different things.

This is not simply an organisational convenience.

It is one of the foundations of computer security.

And Linux gives us an unusually good opportunity to investigate how it works.

Start With the Simplest Question: Who Am I?

Open a Linux terminal and type:

whoami

You might see something such as:

philip

That appears straightforward enough.

But now try:

You may see something similar to:

uid=1000(philip) gid=1000(philip) groups=1000(philip),27(sudo),100(users)

Suddenly there is rather more going on.

Linux does not fundamentally identify you by the word "philip". Internally, users and groups are associated with numerical identifiers.

A user has a UID — a User ID.

A group has a GID — a Group ID.

The names simply make those numbers easier for humans to understand.

Already we have moved beyond the idea that an operating system simply asks for a username and password when it starts.

The operating system is continually asking questions such as:

Who is trying to open this file?
Which groups do they belong to?
Are they allowed to modify it?
Are they allowed to run this program?
Should this process be allowed access to that directory?
Does this user have administrative privileges?

Those checks happen constantly.

You Are Probably Not the Only User

On many Linux systems, you can inspect the user database with:

cat /etc/passwd

Do not be surprised if the result is much longer than expected.

You may find accounts with names connected to services, system processes and applications.

That does not mean dozens of people have secretly been using your computer.

Many of these are system or service accounts.

Linux deliberately allows services to operate as different users because giving every program complete control over the machine would be extremely dangerous.

Imagine a web server.

Does the software serving a website really need permission to edit every personal document on the computer?

Normally, no.

So the operating system can run it with an identity that has only the permissions it actually needs.

This illustrates one of the most important principles in computer security:

Give a user or process only the access it requires.

This is often called the principle of least privilege.

Why Have Different Users at All?

Suppose a computer is shared by three people.

Alice should be able to open Alice's files.

Ben should be able to open Ben's files.

Charlie should be able to open Charlie's files.

There may also be a shared project folder that all three can use.

But none of them should automatically have permission to alter important operating-system files.

Without an access-control system, separating all of this would be extremely difficult.

User accounts provide one layer of that separation.

Groups provide another.

Creating a New Linux User

If you have a Linux machine that you are happy to experiment with — perhaps a Raspberry Pi, spare computer or virtual machine — try creating another user.

On Ubuntu and many Debian-based systems, you can use:

sudo adduser alice

The system will ask you to supply information and choose a password.

On systems using the more general useradd command, you might instead use:

sudo useradd -m alice

The exact administration commands vary slightly between Linux distributions, which is itself a useful reminder that "Linux" is not one single operating system installation.

Now check the account:

id alice

Linux should report Alice's user ID, primary group and any additional groups to which she belongs.

At this point Alice is more than a name on a login screen.

She has become an identity that the operating system can use when making security decisions.

Users Are Useful — Groups Make Them Powerful

Suppose Alice, Ben and Charlie are working on the same programming project.

We could individually configure permissions for each person.

But that quickly becomes awkward.

Instead, we can create a group.

For example:

sudo groupadd programmers

Then add Alice:

sudo usermod -aG programmers alice

We could add Ben and Charlie in the same way.

Now permissions can be granted to the group programmers instead of having to be configured separately for every person.

This is much closer to the way access control works in organisations.

A school might have groups representing:

teachers

students

science_staff

administrators

IT_support

A business might have:

accounts

management

design

marketing

development

When somebody joins or leaves a department, their group membership can be changed rather than thousands of individual files being reconfigured.

Who Owns This File?

Now create an ordinary file:

touch experiment.txt

Then examine it with:

ls -l experiment.txt

You might see:

-rw-r--r-- 1 philip philip 0 Sep 21 14:00 experiment.txt

There is a great deal of information packed into that single line.

Part of it tells us who owns the file.

In this example:

philip philip

The first philip is the owner.

The second philip is the group associated with the file.

But the mysterious sequence at the beginning is particularly interesting:

-rw-r--r--

This describes the permissions.

Decoding Linux Permissions

Ignore the first character for the moment and separate the remaining characters into three groups:

rw- r-- r--

They represent permissions for:

owner

group

others

Each section can contain three letters:

r = read

w = write

x = execute

So:

rw-

means that the owner can read and write the file but does not have execute permission.

The group has:

r--

so members of the group can read it but not modify it.

Everyone else also has:

r--

so they can read the file but cannot change it.

The operating system can therefore make different decisions depending on who is requesting access.

Read, Write and Execute

For files, the meanings are fairly intuitive.

Read (r) means the contents can be viewed.

Write (w) means the contents can be changed.

Execute (x) means the file can be run as a program or script, provided it is otherwise executable.

Directories are slightly more interesting.

For a directory:

read permission allows its contents to be listed;
write permission allows files and directories to be created or deleted within it;
execute permission allows the directory to be entered or traversed.

That distinction is worth experimenting with because directory permissions often surprise students.

Change the Permissions Yourself

Linux provides the chmod command for changing permissions.

For example:

chmod u+x experiment.txt

This adds execute permission for the user who owns the file.

The u means user or owner.

We can also use:

for group

and:

for others.

So:

chmod o-r experiment.txt

removes read permission from everybody classified as "other".

You can combine several changes:

chmod u+rw,g+r,o-rwx experiment.txt

Then use:

ls -l experiment.txt

again and see what changed.

This is one of those computing topics that becomes much clearer when students actually change permissions rather than simply memorising definitions.

Why Do People Write Permissions as Numbers?

You may also encounter commands such as:

chmod 750 program.sh

This looks rather mysterious until you realise that each permission can be represented by a number.

read = 4

write = 2

execute = 1

Add together the permissions you want.

So:

7 = 4 + 2 + 1 = read + write + execute

5 = 4 + 1 = read + execute

0 = no permissions

Therefore:

750

means:

owner = 7 = rwx

group = 5 = r-x

others = 0 = ---

So the resulting permissions are:

rwxr-x---

This numerical form is extremely common in Linux administration.

It is not a different permission system. It is simply another way of expressing the same permissions.

Ownership Matters Too

Permissions make little sense unless the operating system also knows who owns the file.

Linux provides chown for changing ownership.

For example:

sudo chown alice experiment.txt

would make Alice the owner.

We can change both owner and group:

sudo chown alice experiment.txt

Now Alice owns the file and the associated group is programmers.

Imagine that the file contained the source code for a shared programming project.

We could then give Alice full control, allow other programmers to read and modify it, and prevent everybody else from accessing it.

That is access control in action.

A Better Practical: Build a Shared Project Area

Rather than experimenting with a single file, we can make the activity more realistic.

Create a directory:

sudo mkdir /projects

Create a project group:

sudo groupadd projectteam

Add two test users:

sudo adduser alice

sudo adduser ben

Then add both users to the group:

sudo usermod -aG projectteam alice

sudo usermod -aG projectteam ben

Change the group ownership of the project directory:

sudo chown root /projects

Then set suitable permissions:

sudo chmod 770 /projects

The permissions 770 mean:

owner: read, write and execute

group: read, write and execute

others: no access

Now Alice and Ben can both work in the directory because they belong to projectteam.

Another ordinary user should not be able to enter it.

We have effectively created a very small access-control system.

Try to Break Your Own Security

This is where the experiment becomes much more interesting.

Rather than simply checking that something works, deliberately try to make it fail.

Create a file.

Try to modify it as Ben.

Change the group permissions.

Try again.

Remove Ben from the project group.

Try again.

Create another user who is not a member of the group.

Can that user enter the directory?

If not, why not?

Students should start thinking about security as a set of rules that can be tested rather than as a collection of definitions to memorise.

The Most Powerful User: root

Linux has a special administrative account traditionally called:

root

The root user has enormous authority over the system.

It can access files, change ownership, install software, create users, terminate processes and alter the operating system.

That is precisely why normal day-to-day computing should not normally be performed as root.

Instead, many Linux distributions use sudo.

For example:

sudo apt update

or:

sudo adduser alice

The user temporarily requests permission to perform an administrative operation.

This is another example of least privilege.

You do not need unlimited administrative power merely to browse the web, write a document or run Python.

You elevate your privileges only when necessary.

That is a much safer approach.

Why Malware Makes This Important

Imagine downloading a malicious program.

If that program is running with unrestricted administrator privileges, it could potentially change almost anything on the system.

If it is running as a restricted user, the damage it can cause may be limited by the permissions available to that account.

Permissions are not a complete defence against malware, but they form an important part of a layered security model.

This is one reason modern operating systems are increasingly reluctant to let ordinary applications run permanently with administrator privileges.

Windows Does This Too

It is easy to assume that users, groups and permissions are mainly a Linux idea because Linux makes them particularly visible.

Windows has similar concepts.

Open Windows Settings and you will find user accounts.

A Windows account might be:

a local account;
a Microsoft-connected account;
a standard user;
an administrator.

Windows also uses groups.

Two important examples are:

Users

and:

Administrators

A standard account should not automatically have unrestricted control over the system.

When Windows displays a User Account Control message asking whether an application may make changes to your device, you are seeing part of this security structure in action.

Linux Permissions and Windows Permissions Are Not Identical

The traditional Linux model is beautifully simple.

For each file we have permissions associated with:

owner

group

others

Windows, particularly when using NTFS, can use more detailed Access Control Lists or ACLs.

A file might grant different permissions to several individual users and groups.

For example:

Alice: Full control

Project Team: Modify

Teachers: Read

Students: Read

Guest: No access

Linux can also use ACLs, so the real systems are more sophisticated than the simple owner-group-other model suggests.

But the traditional Linux permission system remains an excellent way of understanding the fundamental idea.

The operating system needs to answer:

Who is requesting access, and what are they permitted to do?

A Useful Windows Comparison

Find a file in Windows.

Right-click it and select:

Properties -> Security

You should see users and groups that have permissions associated with that file.

Depending on the file and system configuration, permissions may include:

Full control

Modify

Read & execute

Read

Write

Now compare that with:

ls -l

on Linux.

The interfaces look completely different, but the underlying problem is much the same.

We have:

an object

a user

a set of permitted actions

and an operating system responsible for enforcing the rules.

A Command-Line Windows Investigation

Students who have Windows can also investigate accounts using PowerShell or Command Prompt.

For example:

whoami

works in Windows too.

Try:

whoami /groups

and you can see groups associated with the current security identity.

Another useful command is:

net user

which lists local user accounts.

To investigate file permissions from the command line, Windows provides tools including:

icacls

For example:

icacls example.txt

The result looks rather different from Linux ls -l, but the purpose is related: examining who can do what with a particular file.

What About a Personal Computer?

You might reasonably ask why any of this matters on a computer used by just one person.

There are several reasons.

Your ordinary account should not necessarily have unrestricted administrator access all the time.

Background services do not need access to every personal file.

Applications can operate with limited privileges.

Malware may be restricted by account permissions.

Different services can be isolated from one another.

Sensitive files can be protected.

Network servers can limit which accounts may access resources.

So even a single-user computer is actually a multi-user security environment.

The "users" do not all have to be human.

A Raspberry Pi Makes an Excellent Demonstration

This is an especially good experiment for a Raspberry Pi or spare Linux machine.

You can create several imaginary users without interfering with a student's main computer.

For example:

alice

ben

teacher

student

webserver

You can then create directories and decide who should have access.

Perhaps:

/home/alice

should belong only to Alice.

/schoolwork

might be available to the student and teacher.

/markscheme

might be accessible only to the teacher.

/website

might need to be readable by a web-server account.

Suddenly users and permissions stop being abstract concepts.

They become solutions to real computing problems.

A Challenge for A-Level Students

Try designing a Linux permission system for a fictional school.

Create groups for:

teachers

students

science

computing

administrators

Then create directories representing:

student work

teacher resources

computer-science projects

examination papers

shared resources

Decide which groups should have:

read

write

execute

permissions.

Then implement your design.

Afterwards, test it by logging in as different users.

The important part is not simply getting the commands right.

You should be able to justify every access decision.

Why should a student be able to read one directory but not another?

Why should a teacher be able to modify a file?

Why should an administrator have greater access?

Why should a web server have less?

Those are exactly the sorts of questions real system administrators and cybersecurity professionals have to answer.

One Important Warning

Do these experiments on a machine where changing accounts and permissions will not damage important work.

A Raspberry Pi, virtual machine or spare Linux installation is ideal.

Be particularly careful when using:

sudo

chmod

and:

chown

on important system directories.

Changing permissions recursively on the wrong directory can make a Linux installation unusable.

Learning about security is much more enjoyable when the computer still boots afterwards.

From Examination Topic to Real Operating System

Students often meet access rights as a short section in a computing specification.

They learn phrases such as:

authentication

user account

administrator

file permissions

access rights

And then move on.

But these are not merely examination vocabulary.

They describe mechanisms operating underneath almost everything you do on a modern computer.

When you save a file, launch a program, install software or access something across a network, the operating system may need to decide whether your current identity has permission to perform that operation.

Linux simply makes the machinery unusually easy to see.

The Bigger Lesson

One of the things I like about teaching computing through Linux is that concepts which can seem rather theoretical suddenly become visible.

Instead of merely telling a student that operating systems control access to resources, we can create two users and prove it.

Instead of defining file permissions, we can change them and watch access disappear.

Instead of describing groups, we can create one and use it to control a shared project directory.

And instead of vaguely saying that administrators have greater privileges, we can see precisely what happens when a command requires sudo.

That is far more memorable than simply learning a definition.

Your computer may sit on your desk and be used by only one person.

But inside the operating system is an entire system of identities, ownership, groups and permissions constantly deciding:

Who are you?

What belongs to you?

What are you allowed to do?

Once you understand those questions, you have started to understand not just Linux, but one of the fundamental ideas behind operating systems and computer security.

25 September 2026

Making a Silver-Halide Photogram — When Chemistry Becomes a Photograph

There is something almost magical about watching a photograph appear in a tray of developer.

A blank sheet of photographic paper goes into the liquid. For a few seconds, apparently nothing happens. Then faint grey shapes begin to emerge. Shadows deepen. Edges become clearer. Within a minute or two, an image that simply was not visible before is sitting in front of you.

Of course, it is not magic.

It is chemistry.

And for students who have grown up in a world where taking a photograph means tapping a screen and seeing the result instantly, traditional black-and-white photography provides a wonderful opportunity to connect chemistry, physics, history and technology in one very memorable practical investigation.

I would begin with something extremely simple: a silver-halide photogram.

Then I would take the experiment much further.

I can get an old 35 mm camera out of the cupboard, load it with black-and-white film, let students take photographs and then actually process the film. Instead of an image appearing instantly on a screen, they can watch the entire photographic process unfold in front of them.

The final moment — unrolling the processed film and seeing those tiny negatives for the first time — is particularly satisfying.

For much of the twentieth century, this was how an enormous proportion of family photographs, school photographs, newspaper photographs and scientific images were made.

The smartphone camera may be astonishingly sophisticated, but there is a great deal of science hidden by that convenience.

Traditional photography allows us to uncover it again.

What Is a Photogram?

A photogram is one of the simplest photographs it is possible to make because you do not actually need a camera.

Instead, objects are placed directly onto light-sensitive photographic paper.

The paper is briefly exposed to light and then processed using photographic chemicals.

Where light reaches the paper strongly, the finished image becomes dark.

Where an opaque object blocks the light, the paper remains comparatively light.

Translucent objects produce shades of grey.

The result can be surprisingly beautiful.

Leaves, feathers, mesh, keys, electronic components, pieces of lace, laboratory glassware and even water droplets can create fascinating images.

But behind those shapes lies some excellent chemistry.

The Chemistry Begins with a Precipitate

Traditional photographic materials contain microscopic crystals of silver halides suspended in a gelatin emulsion.

Common examples include silver bromide and silver chloride.

Silver bromide can be formed by precipitation when silver ions meet bromide ions:

Ag+ + Br- -> AgBr(s)

Silver bromide forms a pale cream precipitate.

This gives us a useful introductory experiment before we even touch the photographic paper.

Under appropriate laboratory conditions, students can observe a small-scale precipitation reaction involving silver ions and halide ions and then investigate what happens when the precipitate is exposed to light.

That gives us our first clue about why silver compounds became so important in photography.

They are light sensitive.

Why Does Silver Bromide Respond to Light?

Silver bromide crystals consist of silver ions and bromide ions arranged within a crystal lattice.

When light of sufficient energy is absorbed by the crystal, electrons can become available and ultimately allow tiny quantities of silver ions to be converted into metallic silver.

In simplified terms:

Ag+ + e- -> Ag

Only a tiny amount of metallic silver is initially produced.

There is not enough for us to see a photograph.

Instead, the exposure creates what photographers call a latent image.

The photograph is already encoded into the photographic material, but it remains invisible.

This is one of the ideas I particularly like discussing with students.

An image can exist even though you cannot yet see it.

That is a wonderfully strange concept.

The Developer Reveals the Hidden Image

The job of the photographic developer is to turn that invisible latent image into something we can see.

A developer contains reducing agents.

The tiny regions created by exposure to light encourage further reduction of silver ions within the exposed silver-halide crystals.

More metallic silver is produced.

And metallic silver is dark.

The more exposure a region receives, the greater the eventual density of metallic silver produced during development.

So the image begins to appear.

ILFORD describes conventional black-and-white film in essentially these terms: light creates a latent image within silver-halide crystals, and development amplifies that extremely small initial change into visible grains of metallic silver. Ilford Photo

This is a lovely example of chemistry being used as an amplification process.

A very small photochemical change eventually produces a macroscopic image that our eyes can see.

Watching the Photograph Appear

This is the moment students tend to remember.

The exposed paper goes into the developer.

Initially it looks blank.

Then something happens.

Perhaps the outline of a key appears.

Then the veins of a leaf.

Then areas that were completely exposed become increasingly dark.

It is particularly effective if students have previously only experienced digital photography.

A digital photograph appears instantly because enormously complicated electronics are doing the processing invisibly.

With silver-halide photography, much of the process happens physically in front of you.

You can watch chemistry creating the image.

But Development Cannot Simply Continue

If we left the photographic paper in active developer indefinitely, the image would continue changing.

Traditional processing therefore separates the stages carefully.

A typical black-and-white sequence is:

development;
stop bath or rinse;
fixing;
washing;
drying.

ILFORD's own technical guidance describes film processing as development, stop bath, fixing, washing, wetting-agent rinse and drying, with temperature, timing and agitation important for consistent results. Ilford Photo

This gives students another useful lesson.

Good experimental science is not simply about putting chemicals together.

It is about controlling variables.

Time matters.

Temperature matters.

Concentration matters.

Agitation matters.

And reproducibility matters.

The Fixer Performs a Completely Different Job

Development produces metallic silver in those crystals associated with the latent image.

But a problem remains.

Large amounts of unexposed silver halide are still present.

If we simply took the photograph into daylight at this stage, those remaining light-sensitive crystals would react and the photograph would gradually darken.

We therefore need to remove them.

That is the job of the fixer.

Traditional fixing agents contain thiosulphate ions, usually using sodium thiosulphate or ammonium thiosulphate.

A simplified representation is:

AgBr + 2S2O3^2- -> [Ag(S2O3)2]^3- + Br-

The important point is that the insoluble silver halide is converted into a soluble silver-thiosulphate complex which can be removed from the photographic material.

The metallic silver forming the image remains.

ILFORD similarly describes fixation as the removal of residual silver halide while leaving metallic silver behind to form the permanent image. Ilford Photo

Now the photograph can safely be exposed to ordinary light.

The image has been fixed.

That familiar photographic word therefore has a very literal chemical meaning.

A First Practical: Make a Photogram

For a first session I would keep things deliberately simple.

Students can choose several objects with different optical properties.

For example:

a metal key;
a leaf;
a feather;
a piece of netting;
a glass slide;
a translucent plastic object;
a spring;
wire;
a small electronic circuit board;
pieces of laboratory apparatus.

Under suitable darkroom or safelight conditions, arrange them directly on black-and-white photographic paper.

Expose the arrangement to light for a controlled period.

Then develop, stop, fix and wash the paper according to the photographic paper and chemical manufacturers' instructions.

Suddenly we have something worth investigating rather than merely demonstrating.

Turn the Photogram into a Proper Experiment

There are many variables that students could investigate.

For example, keep everything else constant and change the exposure time.

Try a series such as:

1 second

2 seconds

4 seconds

8 seconds

16 seconds

The exact useful times will depend on the light source, distance and photographic material, so an initial test strip is much better science than simply guessing the "correct" exposure.

Students can compare the resulting density.

Does doubling exposure time double the apparent darkness?

Probably not in any simple visual sense.

And that opens another discussion about photographic response, density and logarithmic scales.

Investigating Distance

Move the light source farther from the photographic paper.

What happens?

Now photography connects with physics.

For an approximately point-like source, illumination is related to distance through the inverse-square relationship:

intensity proportional to 1 / distance^2

Double the distance and, under idealised conditions, the intensity falls to approximately one quarter.

The experiment suddenly connects photochemistry with GCSE and A-level physics.

Opaque, Transparent and Translucent

A photogram can also become a simple investigation of light transmission.

An opaque metal washer may block almost all the light.

Clear glass might transmit most of it.

Frosted plastic scatters the light.

Coloured transparent materials may behave differently depending upon the spectral sensitivity of the photographic paper.

Rather than treating the photograph simply as artwork, students can ask:

What can we infer about an object's interaction with light from the image it produces?

That is a much more scientific question.

Then Bring Out the 35 mm Camera

The photogram establishes the basic principle.

Now comes the part I particularly enjoy.

I can take an old 35 mm camera out of the cupboard.

For many students, even loading the film may be unfamiliar.

Open the back.

Place in the film cassette.

Pull the film leader across.

Engage it with the take-up mechanism.

Close the camera.

Advance the film.

Suddenly photography becomes mechanical as well as chemical.

There is no LCD screen.

There is no instant review.

There is no delete button.

You have perhaps 24 or 36 exposures.

You actually have to think before pressing the shutter.

A Camera Is Really Just Controlling Light

Once the mystery is removed, the basic photographic camera is beautifully simple.

The lens forms an image.

The aperture controls how much light can enter.

The shutter controls how long that light reaches the film.

The film records the image.

That allows us to introduce another basic photographic relationship:

exposure approximately depends on light intensity x exposure time

Photography becomes a practical demonstration of optics.

Students can investigate:

aperture;
shutter speed;
focus;
depth of field;
motion blur;
focal length;
exposure.

Things that are largely automated by a smartphone become visible decisions again.

The Strange Experience of Not Knowing Whether the Photograph Worked

This is something younger photographers have almost completely lost.

Take a photograph digitally and you immediately inspect it.

Was it sharp?

Was the exposure correct?

Did somebody blink?

With film, you do not know.

You press the shutter and move on.

The image exists as a latent chemical change inside the film cassette, but there is nothing yet to inspect.

Only when the film has been developed do you discover what you actually captured.

That delay changes the way you take photographs.

It encourages thought before exposure rather than correction afterwards.

Processing the Film

After the photographs have been taken, the film must be processed.

This provides another wonderful moment.

The film has to be removed from its cassette and loaded onto a processing reel in complete darkness or inside a changing bag.

Once the light-tight processing tank is closed, the rest of the process can normally be carried out in ordinary room light.

Developer is added.

The film is agitated according to the chosen process.

Development is stopped.

The film is fixed.

Then it is washed.

Temperature control matters because photographic development is a chemical reaction and its rate varies with temperature. ILFORD specifically notes the dependence of development on temperature and pH. Ilford Photo

Again, what looks like photography has become experimental chemistry.

And Then Comes the Reveal

Eventually the tank opens.

The film is carefully removed from the spiral.

And there they are.

Tiny photographs.

Except that they look completely wrong.

The bright sky appears dark.

Dark clothing may appear pale.

Light and dark are reversed.

The students are looking at a negative.

For somebody who has only known digital photography, physically holding a strip of 35 mm negatives can be surprisingly fascinating.

The negative is not merely an old-fashioned curiosity.

It reveals how the entire photographic system works.

A bright part of the original scene exposes the film strongly and eventually produces a dense region containing more metallic silver.

A dark part of the scene gives less exposure and produces a more transparent region.

When we subsequently use the negative to make a print, that relationship is reversed again.

The final photograph looks normal.

From Negative to Positive Print

Now we can complete the journey.

Place the negative in an enlarger.

Project its image onto photographic paper.

Adjust focus and enlargement.

Expose the paper.

Then once again:

developer...

the image appears...

stop...

fix...

wash...

dry.

We have gone from:

real scene -> camera -> latent image -> negative -> projected image -> photographic paper -> positive print

That entire chain is enormously instructive.

A modern phone compresses all of this into perhaps a fraction of a second.

The old process allows students to see every stage.

A Contact Sheet Makes an Excellent Teaching Tool

Before making individual enlargements, I would also show students how a contact sheet works.

Place strips of negatives directly against photographic paper beneath glass.

Expose the whole sheet.

Process it.

You then have miniature positive versions of every frame.

This was once an important part of photographic workflow.

The photographer could inspect the contact sheet and decide which frames were worth enlarging.

It also teaches something interesting about selection.

A photographer might take 36 photographs but print only three.

That is another contrast with the modern habit of accumulating thousands of nearly identical digital images.

Why Black-and-White Photography Is Perfect for Teaching Chemistry

Colour photography is scientifically fascinating, but it introduces several additional layers of complexity.

Black-and-white silver photography exposes the essential chemistry far more clearly.

We can follow silver through the whole process.

Begin with:

Ag+

Form:

AgBr

Expose it to light.

Create a latent image.

Develop exposed crystals.

Produce:

Ag metal

Remove unwanted AgBr during fixation.

What remains is an image made largely from microscopic particles of metallic silver.

The photograph is therefore not simply a picture.

It is a chemical object.

There Is Also a Valuable Environmental Discussion

Once we start processing photographic materials, another question becomes important:

What happens to the chemistry afterwards?

Used photographic fixer can contain silver compounds and should not simply be treated as though it were harmless water. ILFORD technical information for photographic processing notes silver in waste fixer streams. Ilford Photo

That creates an opportunity to discuss:

chemical waste;
heavy-metal recovery;
laboratory responsibility;
recycling;
safe storage;
why disposal instructions matter.

This is exactly the sort of broader scientific thinking I want students to develop.

An experiment does not finish simply because we have obtained our result.

We are also responsible for the materials we have used.

Safety Matters

Traditional photography is perfectly capable of being an excellent teaching practical, but it should still be treated as laboratory chemistry.

I would use commercial photographic developer and fixer according to their current instructions and safety data, with appropriate eye protection, gloves where specified, good ventilation and separate labelled equipment.

Silver nitrate used for introductory precipitation experiments requires particular care because it can damage eyes, irritate tissue and produce persistent stains.

Photographic chemistry should never be placed in drinks bottles or unlabelled containers.

And silver-containing waste should be collected and disposed of appropriately rather than casually poured away.

Students should see good chemical practice as part of the experiment, not as an inconvenience added to it.

What Could Students Actually Investigate?

Once the basic technique is working, the possibilities expand enormously.

A photogram could become an investigation into exposure time.

Film could be used to explore shutter speed and motion.

Different apertures could demonstrate depth of field.

A test strip could investigate photographic-paper exposure.

Negatives could be compared for different camera settings.

Students could measure optical density.

They could investigate developer temperature.

They could compare fresh and ageing chemistry.

They could examine film grain under magnification.

They could even compare a film image with a modern digital sensor image of exactly the same scene.

Suddenly one old camera has become a gateway into:

chemistry, optics, electronics, materials science, imaging, measurement, art and technological history.

From Silver Grains to Silicon Pixels

The final stage of the lesson should probably bring us back to the device almost every student has in their pocket.

A smartphone camera does not normally use silver halide.

Its sensor contains millions of photosensitive semiconductor elements.

Photons still have to be detected.

Light still carries the information.

Lenses still have to form an image.

Exposure still matters.

But the method of recording that information has changed dramatically.

In the traditional camera:

light -> chemical change

In a modern digital camera:

light -> electrical signal -> numerical data

That is an extraordinary technological transition.

And it happened within living memory.

"Was Every Photograph Really Made Like This?"

I sometimes tell students that this is how photographs used to be made.

It is worth adding a little historical precision.

Photography has used many processes during its history: daguerreotypes, wet-plate collodion, glass negatives, silver-gelatin materials, colour films and numerous specialist processes.

For example, the daguerreotype used a silver-coated copper plate sensitised with silver halides rather than modern roll film. National Science and Media Museum blog

But for a large part of the twentieth century, silver-halide film and photographic paper were the dominant technology behind ordinary photography.

The family snapshot.

The school photograph.

The holiday photograph.

The wedding album.

News photography.

Scientific photography.

The rolls of film sent away in envelopes and returned days later as prints.

For students who have never known anything except instant digital images, that world can seem surprisingly remote.

Yet it is not ancient history at all.

One Photograph, Several Sciences

This is exactly the sort of experiment I enjoy because it refuses to stay neatly inside one subject.

The precipitation reaction is chemistry.

The photosensitivity is photochemistry.

Development involves reduction.

Fixing involves complex ions and solubility.

The lens introduces optics.

Exposure brings in intensity and time.

The camera introduces engineering.

Film grain leads into materials science.

The history of photography introduces technological change.

And the final print becomes art.

That is much closer to real science than treating every topic as though it belongs in its own isolated chapter of a textbook.

The Photograph Appearing in the Tray Is Still Special

There are faster ways to make a photograph.

There are easier ways.

There are certainly cheaper ways if you already own a smartphone.

But very few are as educational.

I can explain silver ions, precipitation, reduction, lenses, shutter speeds and negatives on a whiteboard.

Students may understand them perfectly well.

But putting a supposedly blank sheet of photographic paper into developer and watching an image slowly emerge is different.

Then taking an old 35 mm camera, processing the film and holding the still-wet strip of negatives up to the light completes the story.

For a few moments, students experience photography not as an app, but as a scientific process.

And perhaps the most important question is no longer:

"What photograph did we take?"

It becomes:

"How did light and chemistry manage to make an image at all?"

That is a much more interesting question.

24 September 2026

What Can a Hole in the Moon Tell Us About Something That Happened Billions of Years Ago?

Look at the surface of the Moon through even a modest telescope and one feature immediately dominates the view.

Craters.

Some are tiny. Others are hundreds of kilometres across. Some overlap older craters. Some have bright rays extending across the lunar surface. Some have relatively smooth floors, while larger examples can contain terraces, collapsed walls and mountains rising from their centres.

They are not simply holes.

They are records of events.

Nobody watched most of these impacts happen. There were no cameras, seismographs or written observations. Yet planetary scientists can examine the crater that remains and work backwards, asking questions such as:

How large was the impacting object?
How energetic was the collision?
At what angle did it arrive?
What was the surface made from?
Which event happened first?
How old might this part of the landscape be?

That makes impact craters a wonderful example of one of the most important ideas in science:

We can investigate events we never actually witnessed by studying the evidence they left behind.

And we can explore some of that science with a surprisingly simple experiment.

Making a miniature impact landscape

The basic experiment needs very little specialised equipment.

I would start with a shallow tray containing a fairly deep layer of fine material such as flour.

On top of the flour, add a very thin layer of contrasting material. Cocoa powder works particularly well, although anything fine and visibly different from the underlying material can be used.

The result represents a very simplified planetary surface.

Then drop an object into it.

A marble or small ball bearing produces an immediate and rather dramatic result.

There is a crater.

There is a raised rim.

Material has been thrown outwards.

The coloured surface layer has been disturbed.

And suddenly there is far more to investigate than simply measuring the diameter of a hole.

Start by changing just one variable

As with any worthwhile scientific investigation, the temptation is to change everything at once.

Resist it.

Choose one variable and investigate it systematically.

For example, keep the impactor the same but release it from heights of:

20 cm

40 cm

60 cm

80 cm

100 cm

After each impact, carefully measure the crater diameter.

Students can record something like:

Drop height	Crater diameter
20 cm	...
40 cm	...
60 cm	...
80 cm	...
100 cm	...

They can then plot crater diameter against drop height.

Immediately the experiment has moved beyond merely producing an impressive photograph.

We are looking for a relationship.

Why should height make a difference?

Before the object is released, it has gravitational potential energy.

For a simple vertical drop:

GPE = mgh

where:

m = mass of the impactor
g = gravitational field strength
h = height above the surface

As it falls, much of that gravitational potential energy becomes kinetic energy.

Immediately before impact:

KE = 1/2 mv^2

The higher the starting point, the greater the energy available when the impactor reaches the surface.

That energy has to go somewhere.

It can:

move surface material;
break or deform material;
eject particles;
heat the impactor and target;
produce sound;
generate vibrations;
and create the crater itself.

Our flour experiment is extremely low-energy compared with a real asteroid impact, but the important principle is there:

An impact is an energy-transfer event.

Mass is another obvious variable

Keep the drop height constant but change the mass of the impactor.

Perhaps students could use objects with similar diameters but different masses.

That is experimentally more interesting than simply changing to a bigger object because it helps separate two different variables:

mass and size.

If the speed is approximately the same, kinetic energy depends directly upon mass:

KE = 1/2 mv^2

Double the mass and, at the same speed, the kinetic energy doubles.

But does the crater diameter double?

Probably not.

And that is where the investigation begins to become much more interesting.

Science is full of relationships that are not simply proportional

Students often meet simple proportional relationships:

double one quantity and another doubles.

Nature frequently refuses to be that cooperative.

Crater dimensions depend on many interacting factors, including:

impact energy;
impactor size;
impactor density;
impact speed;
impact angle;
surface density;
surface strength;
gravity.

Scientists therefore use scaling relationships to connect laboratory experiments, computer simulations and enormous planetary impacts.

A marble falling into flour is obviously not a meteorite hitting the Moon at many kilometres per second.

The experiment is an analogue.

That distinction is important.

We are investigating some of the principles involved in crater formation, not claiming that a tray of flour perfectly reproduces a lunar impact.

That itself is an excellent scientific discussion.

When is a model useful even though it is not completely realistic?

Change the diameter of the impactor

Another investigation is to use spheres of different diameters.

Students might initially predict:

Bigger object = bigger crater.

That is probably true in broad terms, but it raises another question.

Why?

A larger object may also have:

greater mass;
greater surface area;
different density;
different aerodynamic behaviour.

It becomes a nice introduction to experimental design.

If we genuinely want to investigate diameter alone, how do we control the other variables?

This is often more scientifically valuable than producing a perfectly neat graph.

Students begin discovering that designing a fair experiment can be harder than carrying one out.

What happens if the impactor arrives at an angle?

Dropping objects vertically is easy.

Real objects in the Solar System are not obliged to cooperate.

Asteroids and meteoroids can approach a planetary surface at different angles.

A simple classroom experiment can investigate this by arranging for the projectile to enter the material obliquely rather than vertically.

Students can investigate:

crater shape;
crater length and width;
direction of ejecta;
distribution of disturbed surface material.

At the relatively low velocities of a classroom experiment, changing the angle may produce noticeably asymmetric results.

Real planetary impacts are considerably more complicated because they usually occur at enormous speeds. Hypervelocity impacts can behave rather differently from a slowly dropped ball.

Again, that difference provides an excellent opportunity to discuss the limitations of models.

Look at the ejecta, not just the crater

This is one reason I particularly like using a thin contrasting surface layer.

When the impact occurs, material is thrown outwards.

This material is called ejecta.

Instead of simply measuring crater diameter, students can look at:

maximum ejecta distance;
direction;
symmetry;
thickness;
streaks or rays;
distribution around the crater.

Photographing the tray directly from above makes these patterns much easier to compare.

A ruler included in each photograph gives a scale.

Students could even analyse the images digitally rather than measuring the crater directly.

Suddenly we have moved into scientific imaging and quantitative image analysis.

High-speed video could make this even better

This is one experiment where a camera can reveal something the eye easily misses.

Film the impact at the highest useful frame rate available.

Played back slowly, students may see:

the impactor entering the surface;
material beginning to move outwards;
the developing cavity;
ejecta travelling away from the impact;
material falling back around the crater.

What appears to be an instantaneous event becomes a sequence.

A side view can show the ejecta rising.

A top view reveals its distribution.

Using two cameras simultaneously would make an especially effective demonstration because the same event could be examined from two completely different perspectives.

A crater is much more than a hole

Now we can return to the Moon.

Planetary scientists do not simply measure crater diameters.

The morphology of a crater — its shape and structure — contains information.

Depending on its size and the conditions under which it formed, an impact crater can contain features such as:

a raised rim;
an ejecta blanket;
rays extending across the surrounding terrain;
slumped or terraced walls;
a relatively flat floor;
central peaks in larger complex craters.

These structures tell us something about the extraordinary forces involved.

For a sufficiently large impact, the ground does not simply behave like a rigid solid being struck with a hammer. Under the enormous pressures produced during a hypervelocity impact, rock can fracture, flow and rebound on a huge scale.

That is why enormous impact structures can be far more complicated than simple bowl-shaped holes.

Why does the Moon have so many craters?

The Moon provides an almost perfect place to introduce another geological idea.

A crater can only tell us its history if the evidence survives.

On Earth, landscapes are continually being modified.

We have:

wind;
rain;
rivers;
glaciers;
vegetation;
weathering;
erosion;
sedimentation;
plate tectonics.

Earth's surface is extraordinarily active.

The Moon has no rivers washing craters away, no vegetation covering them and no active plate tectonic system recycling its surface in the way Earth's crust is recycled.

Its landscape can therefore preserve extremely old evidence.

Looking at the Moon is rather like looking at an ancient astronomical archive.

Counting craters can even tell us something about age

Imagine two neighbouring lunar surfaces.

One is covered with craters.

The other contains relatively few.

Which is probably older?

The heavily cratered surface has generally been exposed to impacts for longer, whereas a younger surface may have been resurfaced more recently.

Planetary scientists therefore use crater counting as one technique for comparing the relative ages of surfaces.

It is not simply:

more craters = exact age.

Scientists must consider crater sizes, resurfacing events, overlapping structures and models of impact frequency.

But the central principle is wonderfully accessible.

If impacts accumulate with time, the number and distribution of craters can help reconstruct a landscape's history.

A geological detective story

Overlapping craters introduce another beautifully simple idea.

Suppose crater A cuts across crater B.

Which formed first?

Crater B must already have existed before crater A could have disrupted it.

Students have just used relative dating.

The same reasoning is used throughout geology.

A feature that cuts another feature must generally be younger than the feature it cuts.

A tray of flour has now taken us into stratigraphy and geological history.

Mars adds another layer to the story

The same reasoning can be applied to Mars.

But Mars has had a different geological and atmospheric history from the Moon.

Its surface shows:

impact craters;
enormous volcanoes;
valleys;
sedimentary structures;
evidence of erosion;
ancient surfaces;
younger resurfaced areas.

Comparing cratered landscapes on Mars with those on the Moon therefore becomes much more than identifying holes.

Students can ask:

What has happened to this landscape since the crater formed?

Has material filled the crater?

Has erosion modified it?

Has volcanic activity covered older structures?

Has wind moved sediment across it?

This is planetary geology becoming a genuine investigation rather than simply learning the names of planets.

And then there is Earth

Impact craters exist here too.

They are simply harder to preserve.

One of the most famous impact structures is associated with the event about 66 million years ago at the end of the Cretaceous Period.

The Chicxulub impact structure in what is now Mexico is roughly 180 km across.

Its significance reaches far beyond geology because it is associated with one of the greatest mass-extinction events in Earth's history.

Suddenly our tray of flour connects:

physics
to astronomy
to geology
to palaeontology
to evolution.

That is exactly why I enjoy experiments that sit outside the formal syllabus.

Individual school subjects suddenly stop looking quite so separate.

Could students calculate the impact energy?

Yes — and this could make an excellent A-level extension.

For the falling object, begin with:

GPE = mgh

If losses are ignored, immediately before impact:

KE approximately equals mgh

Students could calculate the approximate impact energy for each drop.

They could then plot:

crater diameter against impact energy

rather than simply crater diameter against height.

This is scientifically much more meaningful because different combinations of mass and height can produce the same gravitational potential energy.

For example, students could deliberately choose different masses and heights designed to give approximately equal values of mgh.

Would they produce identical craters?

That becomes a much more sophisticated investigation.

A useful challenge: equal energy, different impactor

Suppose we arrange two impacts with approximately the same calculated energy.

One uses:

a lighter object dropped from higher up.

The other uses:

a heavier object dropped from a lower height.

If KE is approximately the same, will the craters be identical?

That question is far more interesting than merely confirming that higher drops make larger holes.

Students may discover that impactor geometry, momentum, contact area and the behaviour of the target material also matter.

The experiment begins to reveal the danger of reducing a complicated physical event to a single number.

Momentum gives us another way of looking at it

Kinetic energy is not the only useful quantity.

Momentum is:

p = mv

Two objects can have the same kinetic energy but different momenta.

This gives A-level students another possible investigation.

Which quantity appears to correlate more strongly with the crater dimensions in our particular experimental setup?

Energy?

Momentum?

Impactor diameter?

Perhaps no single variable completely explains the result.

That is much closer to real experimental science.

An investigation students could genuinely design themselves

I would be tempted not to give students a complete method.

Instead I might provide the question:

What determines the size of an impact crater?

Then allow them to decide:

what variable to change;
what quantities to measure;
what controls are necessary;
how many repeats are needed;
how uncertainty should be handled;
what graph should be plotted.

Different students might investigate entirely different aspects of the same phenomenon.

One group could investigate mass.

Another could investigate height.

Another could investigate projectile diameter.

Another could concentrate on impact angle.

Another could analyse ejecta.

At the end, the class could combine its evidence.

That begins to resemble the way scientific research actually develops.

Repeats matter

Flour does not behave perfectly.

Neither do students dropping marbles.

Two apparently identical impacts may produce slightly different crater diameters.

That is not experimental failure.

It is experimental reality.

Repeat each condition several times and calculate a mean crater diameter.

Students can then discuss:

random variation;
anomalous results;
measurement uncertainty;
repeatability;
how many repeats are sufficient.

A very visually dramatic experiment has quietly become an exercise in serious experimental technique.

One practical problem: how do you measure a crater?

Even this apparently simple question deserves thought.

Where exactly does the crater end?

Do we measure:

the inner depression?
the outer rim?
the maximum diameter?
two perpendicular diameters and take a mean?

If the crater is elliptical, one measurement is clearly inadequate.

For an angled impact, students might record:

major axis = ...

minor axis = ...

and calculate their ratio.

Experimental definitions matter.

Two groups cannot meaningfully compare their data unless they have agreed what they mean by "crater diameter".

That is a lesson extending far beyond planetary science.

Improve the experiment with photography

A particularly good method would be to create a permanent visual record of every impact.

Mount a camera above the tray.

Keep:

camera position;
focal length;
lighting;
tray position;
scale ruler

constant.

Photograph every crater before resetting the surface.

The photographs can then be compared later.

Students could measure crater dimensions directly from the image and perhaps investigate the area covered by ejecta.

A numbered card beside the tray could identify each experimental condition.

That turns a messy practical experiment into a much better documented investigation.

Safety and practical organisation

The experiment is straightforward, but a little organisation helps.

Use relatively small, manageable impactors and sensible drop heights.

Protect the surrounding area because fine powders can travel surprisingly far.

Avoid throwing hard objects or launching high-speed projectiles.

The aim is to investigate impact processes, not to reproduce genuine asteroid velocities in the laboratory.

A large tray or shallow container also makes resetting the surface much easier.

After each test:

recover the impactor;
level the flour;
recreate the thin contrasting layer;
check the scale;
repeat the experiment.

Consistency here will greatly improve the results.

The experiment I would like students to remember

The best science practicals are not necessarily those involving the most complicated equipment.

Sometimes the best experiment begins with a question that becomes larger the longer you investigate it.

Drop a marble into flour and initially the question is:

How big is the hole?

A few minutes later it becomes:

How does crater diameter depend on impact energy?

Then:

Can we infer the properties of an impactor from the crater it leaves behind?

And eventually:

How can scientists reconstruct an event that happened billions of years before human beings existed?

That is a remarkable journey from a baking ingredient and a marble.

Science is the art of reading evidence

Perhaps that is the most important idea behind this experiment.

Science is not restricted to events we can watch happening.

We cannot travel back to observe the formation of every lunar crater.

We cannot stand beside an asteroid as it strikes ancient Mars.

We were not present for the enormous impacts that shaped the early Solar System.

But those events left evidence.

Crater dimensions.

Ejecta.

Fractured rocks.

Overlapping structures.

Chemical signatures.

Altered landscapes.

Scientists learn to read those clues.

And from them, we reconstruct a history.

So the next time you look through a telescope and see the battered surface of the Moon, it is worth remembering:

You are not simply looking at holes in the ground.

You are looking at billions of years of Solar System history, written into the landscape.