Investigating Refraction and Critical Angle with a Semicircular Block

Refraction is one of the most important topics in GCSE and A Level Physics. A simple semicircular acrylic block, a ray box, and a protractor create one of the clearest experiments for observing how light bends, how angles relate to each other, and where total internal reflection begins.

This investigation connects the theory of refractive index with hands-on measurements and gives students real data to support Snell’s Law.

---

Why Use a Semicircular Block?

The semicircle has a special advantage:
If the light ray enters through the curved surface, it always hits the flat surface at a right angle, meaning no refraction occurs at entry.

This ensures all bending happens at the flat face, simplifying measurements and removing unnecessary complications.

---

The Experiment

Equipment:

• Semicircular Perspex or glass block

• Ray box with single-slit attachment

• Protractor or printed angle sheet

• A3 paper and pencil

• Ruler

---

1. Investigating Refraction (Snell’s Law)

1. Place the block on paper and draw around it.

2. Shine a narrow ray into the curved surface so it reaches the flat side at different angles of incidence.

3. Mark the incident ray, refracted ray, and normal.

4. Measure:

   • Angle of incidence $i$

   • Angle of refraction $r$

5. Plot a graph of $\sin i$ against $\sin r$.

The gradient of the straight-line graph gives the refractive index of the block.

Typical value for Perspex: 1.49.

---

2. Investigating Critical Angle and Total Internal Reflection

1. Keep the light ray inside the block and slowly increase the angle of incidence at the flat face.

2. Observe:

   • At small angles → refraction out of the block

   • At a specific angle → refracted ray emerges at 90°

   • Beyond that → the ray reflects back internally

That angle where the refracted ray is at 90° is the critical angle $c$.

From measurements:

$$\sin c = \frac{1}{n}$$

For Perspex

$$c \approx 42^\circ$$

Students can test this experimentally and compare to theory.

---

What Students Learn

• Light changes speed when entering a new medium

• Snell’s Law links angles and refractive index

• Total internal reflection occurs beyond the critical angle

• Semicircular blocks make the geometry clean and accurate

They also gain practice drawing diagrams, measuring angles, and producing graphs — essential skills for GCSE and A Level exams.

---

Skills Highlight

• Accurate angle measurement

• Collecting data for $\sin i$ vs $\sin r$

• Calculating refractive index

• Identifying the critical angle

• Understanding when and why total internal reflection happens

---

Why It Works in Teaching

The experiment is fast, visual, and precise. Students see the ray bend in real time, compare theory with measurement, and consolidate one of the most important optical concepts in physics — with equipment found in every school lab.

Measuring the Rate of an Enzyme – Amylase and Starch

 

GCSE Biology

Measuring the Rate of an Enzyme – Amylase and Starch

Enzymes are biological catalysts — they speed up reactions inside living organisms without being used up. One of the simplest and most reliable experiments at GCSE Biology measures the rate at which amylase breaks down starch into maltose.

This practical introduces students to reaction rates, variables, and the principles of enzyme action, such as temperature, pH, and substrate concentration.

---

The Science Behind the Practical

Amylase is an enzyme produced in the salivary glands and pancreas.
Its job is to catalyse the breakdown of starch, a long-chain carbohydrate, into maltose, which can be further digested into glucose.

The key ideas students learn:

• Enzymes have an active site where substrates bind

• They have an optimum temperature and pH

• High temperatures denature enzymes

• Reaction rate can be measured by tracking the disappearance of starch

The amylase–starch experiment is a perfect way to bring these concepts to life.

---

The Practical

Equipment:

• Amylase solution

• Starch solution

• Iodine in potassium iodide

• Spotting tile

• Water bath

• Pipettes

• Stopwatch

• Beakers/test tubes

---

Method

1. Warm amylase and starch solutions in a water bath to the chosen temperature.

2. Mix a set volume of amylase with a set volume of starch and start the stopwatch.

3. Every 10 or 20 seconds, place a drop of the reaction mixture onto a drop of iodine in a spotting tile.

4. Iodine turns blue–black in the presence of starch.

5. Continue sampling until the iodine no longer changes colour.

6. Record the time taken for starch to disappear.

7. Repeat at different temperatures or pH levels for comparison.

The shorter the time taken, the faster the reaction rate.

---

Typical Results

Effect of Temperature on Amylase Activity

Temperature (°C)	Time for Starch to Disappear (s)	Relative Rate
0	180	Slow
20	70	Moderate
37	30	Fast (optimum)
60	200	Very slow (enzyme partially denatured)
80	No reaction	Denatured

Students clearly see that amylase works fastest at human body temperature (~37°C) and slows dramatically when heated or cooled.

---

Variables Students Control

• Independent variable: temperature / pH/enzyme concentration

• Dependent variable: time for starch to disappear

• Controlled variables: volume of solutions, concentration, water bath conditions, sampling interval

This helps build solid, practical, and exam skills.

---

Skills Highlight

• Using iodine to test for starch

• Measuring reaction rate via the disappearance of the substrate

• Controlling variables for fair testing

• Drawing graphs of rate vs temperature or pH

• Linking data to enzyme structure and denaturation

---

Why It Works in Teaching

The amylase practical is simple, visual, and meaningful. Students watch a colour change disappear, linking biological theory with chemical testing. The experiment also prepares them for practical questions in GCSE exams and strengthens their understanding of enzymes at work inside the digestive system. Many students are amazed at how fast enzymes work.

Visual Inattention – Gorillas in Our Midst and How Magic Tricks Work

 

A Level Psychology

Visual Inattention – Gorillas in Our Midst and How Magic Tricks Work

One of the most famous studies in psychology is Simons and Chabris’ “Gorillas in Our Midst”.
In this experiment, participants watched a video of people passing a basketball and were asked to count the passes. Half of the viewers failed to notice a person in a full gorilla suit walking across the screen.

This striking demonstration shows inattentional blindness — the failure to see something obvious when attention is focused elsewhere.
It’s not a flaw in our eyes, but a limitation of our cognitive attention system.

This same psychological principle explains why magicians can make objects disappear, switch items unnoticed, or produce illusions that seem impossible. Magic works because our brains prioritise, filter, and ignore far more than we realise.

---

What Is Inattentional Blindness?

Inattentional blindness happens when:

• attention is focused on a demanding task,

• the unexpected event is unrelated to that task, and

• the person has no reason to expect anything unusual.

The gorilla walking across the screen is visible to the eyes but invisible to attention.

This phenomenon tells us that perception is active, not passive. We don’t see the world fully — we see what we are paying attention to.

---

Why Do So Many People Miss the Gorilla?

Psychology research shows several factors increase inattentional blindness:

1. High cognitive load

When mental effort is focused on counting, solving, or tracking, fewer resources remain for noticing the unexpected.

2. Expectations

People expect only basketball-related events. A gorilla simply isn’t anticipated.

3. Expertise and familiarity

Those familiar with selective attention tasks, such as elite sports players, are sometimes more likely to notice unusual stimuli — or sometimes less likely, depending on what they focus on.

4. Change blindness links

Even when looking directly at something, rapid or unexpected changes often go unnoticed.

Magicians use all of these factors to their advantage.

---

How Magic Tricks Exploit Inattentional Blindness

Illusionists understand attention better than most psychologists. Many magic effects rely on:

1. Misdirection

The magician draws your attention to the right hand, while the left hand performs the method.
Your eyes may see it — your attention does not.

2. Expectation violation

If an object has behaved consistently throughout the trick, your brain stops monitoring it closely.
This makes it perfect for a switch or disappearance.

3. Cognitive overload

Fast movements, patter, humour, noise, or a sudden surprise occupy working memory, leaving fewer resources to notice the deception.

4. Attentional “bottlenecks”

The brain cannot consciously process everything at once.
Magicians create moments where only one interpretation seems possible — and hide the real method just outside the spotlight of attention.

Students recognise how the same cognitive limitations that hide the gorilla also hide the secret of a magic trick.

---

Why This Topic Works in A Level Psychology

Inattentional blindness links directly to:

• selective attention

• cognitive load

• perception and information processing

• real-world consequences (driving, eyewitness testimony, health and safety)

• applications in advertising, sports, and UX design

It shows students that what we think we saw may not match what actually happened — a key theme in cognitive psychology.

---

Skills Highlight

• Evaluating Simons & Chabris (method, validity, ethics, conclusions)

• Linking attention theories to everyday behaviour

• Analysing real-world failures of perception

• Understanding how attention can be manipulated

 

Understanding Encryption – Writing a Caesar Cipher in Python

Encryption is at the heart of modern cybersecurity — from messaging apps to online banking. Students often imagine encryption as something complex and mysterious, but many key ideas begin with surprisingly simple methods. One of the earliest examples is the Caesar cipher, used by Julius Caesar to send secure messages to his generals.

Writing a Caesar cipher in Python is an excellent introduction to encryption at GCSE and A Level Computing. It helps students understand substitution ciphers, modular arithmetic, character encoding, and the logic behind more advanced systems.

---

What Is a Caesar Cipher?

A Caesar cipher shifts each letter in a message by a fixed number of positions in the alphabet.
For example, with a shift of 3:

• A → D

• B → E

• C → F

The message “HELLO” becomes “KHOOR”.

It’s simple, but it introduces students to two key ideas:

• Encryption (scrambling a message)

• Decryption (undoing the scrambling)

Modern encryption is vastly more complex — but the logic of substitution and key-based security begins here.

---

Writing a Caesar Cipher in Python

Here is a simple encryption function:

def caesar_encrypt(text, shift):
    result = ""
    for char in text:
        if char.isalpha():
            base = ord('A') if char.isupper() else ord('a')
            result += chr((ord(char) - base + shift) % 26 + base)
        else:
            result += char
    return result

And a matching decryption function:

def caesar_decrypt(cipher, shift):
    return caesar_encrypt(cipher, -shift)

Students can test their program:

message = "Secret Message"
encrypted = caesar_encrypt(message, 4)
decrypted = caesar_decrypt(encrypted, 4)

This shows encryption and decryption clearly and logically.

---

Extending the Task

More confident students can:

• Add support for punctuation and numbers

• Create a brute-force attack to test all 26 possible shifts

• Analyse letter frequencies to understand why the cipher is weak

• Link this to modern encryption and hashing algorithms

This builds understanding of cybersecurity, algorithm design, and ethical hacking.

---

Why It Works in Teaching

Students gain experience in:

• String manipulation

• Loops and conditionals

• Character encoding (ASCII/Unicode)

• Modulus arithmetic

• Thinking like both a programmer and an attacker

Most importantly, they see that encryption is not magic — it’s a series of logical steps designed to hide information.

 

Testing Water Quality – Hardness and pH

GCSE Chemistry

Water may look clean, but its chemical properties vary widely depending on geology, treatment, and environmental factors. Two of the most important measures students learn at GCSE are water hardness and pH. These tests show how dissolved ions affect everyday life — from limescale in kettles to how soap lathers in hard or soft water.

---

What Is Hard Water?

Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
These ions come from rocks such as limestone, chalk, and dolomite as rainwater slowly dissolves them.

Hardness affects:

• how well soap lathers

• the formation of limescale

• water taste

• efficiency of kettles, boilers, and washing machines

Testing hardness gives students a direct link between chemistry and household science.

---

The Soap Solution Test (GCSE Core Practical)

Equipment:

• Water samples (tap water, bottled water, distilled water, rainwater, river water, pond water, seawater)

• Standard soap solution

• Conical flasks

• Measuring cylinder

Method:

1. Place 10 cm³ of water into the flask.

2. Add soap solution a few cm³ at a time, shaking well.

3. Measure how much soap is needed to form a stable lather for 10 seconds.

4. Repeat for each water sample.

Interpretation:

• More soap needed → harder water

• Less soap needed → softer water

This test works because Ca²⁺ and Mg²⁺ ions react with soap to form scum, reducing lather.

---

Testing pH

pH tells us how acidic or alkaline water is. Most drinking water is pH 6.5–8.5, depending on treatment and natural minerals.

Methods:

• pH paper (quick, approximate)

• Universal indicator (colour scale)

• Digital pH sensor (accurate, ideal for A-level or more precise investigation)

Causes of variation:

• Dissolved carbon dioxide

• Natural mineral content

• Pollution or acid rain

• Water treatment chemicals (e.g. chlorine)

Students can compare pH values across water sources and relate differences to geology and human activity.

---

Typical Classroom Results

Water Sample	Soap Added for Lather (cm³)	Hardness	pH
Distilled water	1–2	Very soft	~7
Local tap water	4–6	Moderately hard	7.5
Bottled spring water	6–8	Hard	7
Rainwater	1–2	Soft	5.5–6 (slightly acidic)

Students immediately see why some regions suffer from limescale — and why rainwater can be acidic despite looking clean.

---

Skills Highlight

• Performing fair comparative tests

• Measuring and recording pH values

• Interpreting data from qualitative and quantitative methods

• Understanding ions in solution and their effects on everyday life

---

Why It Works in Teaching

These tests connect GCSE Chemistry directly to real life. Students recognise the science behind household appliances, water treatment, soap use, and environmental issues — making the topic both relevant and memorable.

 

Simple Harmonic Motion – Measuring SHM with PASCO Sensors

Simple Harmonic Motion (SHM) appears all over physics: oscillating springs, swinging pendulums, vibrating masses, tuning forks, air columns, and even molecules in solids. It’s a perfect topic for hands-on investigation, and with PASCO sensors, students can collect precise displacement, velocity, and acceleration data to see SHM unfold in real time.

---

What Is Simple Harmonic Motion?

An object in SHM experiences a restoring force that is proportional to its displacement and acts towards equilibrium:

$$F = -kx$$

This produces motion that is:

• periodic,

• symmetrical,

• and modelled by sine and cosine functions.

PASCO equipment makes these ideas visible and measurable.

---

Measuring SHM with PASCO Sensors

1. Spring–Mass System (Wireless Motion Sensor or Smart Cart)

Attach a mass to a vertical or horizontal spring.
Start oscillations and use the motion sensor to track displacement.

Data shows:

• sinusoidal displacement–time graphs

• velocity 90° out of phase

• acceleration proportional to –displacement

Students calculate the period:

$$T = 2\pi \sqrt{\frac{m}{k}}$$

and verify it experimentally.

Add the Force Sensor to see the effect of force.

---

2. Simple Pendulum (Motion Sensor or Photogate)

A PASCO rotational sensor or a motion sensor or photogate can track the oscillation period of a small pendulum.
Students test how the period changes with:

• length of the string

• amplitude (for small angles)

and compare data with:

$$T = 2\pi\sqrt{\frac{L}{g}}$$

---

3. Smart Cart Oscillations on a Track

The PASCO Smart Cart, acting as a mass attached to long springs, provides a clean horizontal SHM system. The Pasco Track is mounted at a steep angle, and the cart is allowed to oscillate, suspended by a spring.

With the track level two, springs can be used, one at the top and the other at the bottom.
Built-in position and acceleration sensors allow simultaneous measurement of:

• $x(t)$

• $v(t)$

• $a(t)$

Graphs clearly show the phase relationships between each.

---

4. Torsional Oscillator (Rotary Motion Sensor)

Using a rotary motion sensor and a torsion wire, students observe rotational SHM.
They can measure moment of inertia, torsion constant, and compare with:

$$T = 2\pi\sqrt{\frac{I}{k_\tau}}$$

This links SHM theory to rotational dynamics.

---

Why PASCO Makes SHM Clear

• Real-time graphs reveal phase differences instantly

• Data is smooth and accurate, ideal for curve fitting

• Students can test how mass, stiffness, and amplitude affect period

• Results link directly to A Level equations and modelling

The combination of hands-on systems and digital sensors helps students understand SHM as both a physical motion and a mathematical model.

---

Skills Highlight

• Collecting and analysing real-time motion data

• Using PASCO sensors to measure displacement, velocity, and acceleration

• Fitting sinusoidal curves to experimental data

• Investigating how system parameters affect oscillation

• Linking mathematical models to physical behaviour

---

Why It Works in Teaching

SHM is everywhere — from clocks and guitars to earthquakes and resonance.
PASCO technology lets students see the full picture:
the forces, the curves, the timing, and the mathematics behind oscillatory systems.

 

Exploring Correlation – Do Taller People Have Bigger Feet?

Correlation is one of the first statistical ideas students meet at GCSE and A Level, and it’s much easier to understand when linked to real-world data. One of the simplest — and most popular — classroom investigations asks a surprisingly sensible question:

Do taller people have bigger feet?

It’s a fun way to explore data collection, scatter graphs, line of best fit, and the difference between correlation and causation.

---

The Investigation

Students work in pairs or groups to collect two sets of data:

• Height (in cm)

• Foot length (in cm or shoe size converted to cm for accuracy)

Measurements are plotted on a scatter graph, with height on the x-axis and foot size on the y-axis.

A line of best fit allows students to see whether a pattern exists — and in most cases, the answer is yes. Taller people tend to have longer feet.

---

What Students Discover

1. Correlation, Not Causation

A positive correlation does not mean one variable causes the other. Height doesn’t cause big feet, and big feet don’t cause height — both are linked by underlying factors such as genetics and growth.

2. Strength of Correlation

Students calculate or estimate:

• Weak correlation

• Moderate correlation

• Strong correlation

• Very strong correlation

Real biological variation means the points never fall perfectly on the line.

3. Outliers Matter

Some individuals don’t follow the general pattern. Discussing why — genetics, age differences, measurement error — helps students think critically about real data.

4. Regression Line Interpretation

The line of best fit helps predict approximate values, but with caution. Prediction inside the data range (interpolation) is reasonable; outside it (extrapolation) becomes unreliable.

Yes, taller people generally have larger feet because there is a positive correlation between height and foot size. However, this relationship is not absolute, as genetics, nutrition, age, and sex also influence foot size, leading to individual variation. Some taller people have average-sized feet, while some shorter people have large feet. 

---

Classroom Skills Developed

• Collecting measurable, reliable data

• Plotting accurate scatter graphs

• Drawing and interpreting lines of best fit

• Understanding correlation coefficients

• Evaluating outliers and data reliability

---

Why It Works in Teaching

Students enjoy this investigation because it’s personal, measurable, and immediately meaningful.
The data isn’t abstract — it comes from them.

This makes the statistical concepts far easier to grasp and shows how correlation helps scientists, medics, economists, and businesses understand patterns in the world.