Experiments with a PASCO Light Sensor (and why light is never ‘just light’)

If you’ve ever said, “It’s brighter over there,” congratulations — you’ve made a scientific observation. If you’ve ever argued with someone about it, you’ve made a scientific dispute. The PASCO Light Sensor is the peace treaty: it turns “bright” into numbers you can graph, analyse, and (most importantly) use to win the argument politely.

In this post I’ll share a set of simple, reliable experiments you can run with a PASCO Light Sensor (in class or at home), plus what to measure, what to plot, and the usual “why do my results look odd?” troubleshooting.

What you’ll need

PASCO Light Sensor (any PASCO light/illuminance sensor)
PASCO interface / SPARKvue or Capstone (or whatever you’re using)
A lamp (desk lamp is fine) and/or torch
Metre rule or tape measure
A sheet of white paper or card (as a reflector)
Optional: coloured filters/cellophane, sunglasses, polarising sheets, diffraction grating, blinds/curtains

Tip: Try to keep room lighting constant. Daylight through a window is lovely… and also a chaos agent.

Experiment 1: The inverse square law (the “physics that actually works” one)

Question: How does light intensity change with distance from a point source?

Method

Set the lamp at one end of a bench. Keep it still throughout.
Place the sensor facing the lamp.
Measure distance $d$ from the lamp to the sensor (start at, say, 10 cm, then 15, 20, 30, 40, 50 cm).
Record light intensity at each distance (lux).
Repeat each reading 2–3 times and average.

What to plot

Plot Intensity (lux) vs distance (m) → curve
Plot Intensity (lux) vs 1/d² → should be a straight line (ish!)

What you should find
If the lamp behaves like a point source, intensity $\propto 1/d^2$ .

Common problems

At small distances the lamp isn’t a point source.
If the sensor saturates, move further away or reduce brightness.
Reflections from walls and benches can lift the readings.

Experiment 2: Absorption and transmission (a.k.a. “how good are your sunglasses?”)

Question: How much light gets through different materials?

Method

Fix the lamp and sensor positions (don’t change distance).
Record baseline intensity $I_0$ .
Place a material between lamp and sensor (paper, tracing paper, plastic, sunglasses, tinted film, acetate).
Record transmitted intensity $I$ .
Calculate percentage transmission:

\%T = (I/I_0)\times 100

Extensions

Stack layers of the same material and see if transmission drops steadily.
Compare clear vs frosted plastic.
Compare different “SPF” sunglasses (if available).

Experiment 3: Reflection — colour, surface, and angle (the “why is it brighter off the white card?” one)

Question: What affects reflected light intensity?

Method

Put a white card on the bench.
Shine a lamp or torch at it at a fixed angle.
Point the sensor towards the card to measure reflected light.
Compare different surfaces: white paper, coloured paper, foil, matte card, glossy magazine.

Ideas to test

Colour: Which reflects more: white, yellow, red, black?
Texture: Glossy vs matte
Angle: Rotate the card and see how reflections change

Bonus physics
Specular reflection (mirror-like) vs diffuse reflection (scattered). Foil behaves very differently from paper.

Experiment 4: Light intensity and shadow patterns (because shadows have structure)

Question: How does intensity change across a shadow?

Method

Place a small object (ruler, pencil, hand) between lamp and sensor.
Move the sensor slowly sideways across the shadow edge.
Record intensity at each position.

What to plot

Intensity vs position → you’ll see a drop, then a rise.
With a small light source you get a sharp edge; with a bigger source you get a fuzzy “penumbra”.

Link to GCSE Physics
This is a brilliant way to measure umbra and penumbra rather than just draw them.

Experiment 5: Flicker and mains lighting (why 230 V lighting isn’t steady)

Question: Do lights actually stay constant?

Many LED lights and some fluorescents flicker at 100 Hz (UK mains is 50 Hz; brightness often varies twice per cycle).

Method

Put the sensor under a mains-powered lamp (not daylight).
Record intensity vs time at a high sample rate (if possible).
Look for periodic variation.

What you’ll see
Some lights are smooth; others are “invisible strobe lights”. Great discussion for cameras, headaches, and why slow-motion video looks odd under certain lights.

Experiment 6: Polarisation (if you have polarising filters)

Question: How does light intensity change through crossed polarisers?

Method

Place one polariser in front of the light source (or in front of sensor).
Place a second polariser in front of it.
Rotate one polariser and record intensity at angles 0° → 90°.

What to plot

Intensity vs angle (°)

Expected pattern
It follows Malus’ Law:

I = I_0 \cos^2(\theta)

(And yes, it’s one of those rare laws that behaves beautifully in a school lab.)

Data handling (make it look like proper science)

Take repeats and average.
Control one variable at a time (distance OR filter OR angle — not all at once).
Use units carefully (lux, metres, degrees).
Try at least one linearised graph (e.g., intensity vs $1/d^2$ ).

Troubleshooting: the “why are my readings weird?” section

Sunlight changed → close curtains or work at night (science is glamorous).
Reflections → move away from white walls, use a dark cloth behind.
Sensor orientation → keep it facing the same way each reading.
Auto-ranging → if the sensor/interface changes range, it can look jumpy. Lock range if possible.

If you’re teaching this (or revising)

These experiments are brilliant for:

GCSE Physics: inverse square, reflection/absorption, shadows
A-Level Physics: Malus’ law, measurement uncertainty, data linearisation, practical write-ups

And they’re perfect for filmed demonstrations too — you can see the graph change live, which is exactly the kind of “Ohhh, I get it” moment students remember.

19 February 2026