Physics 11–12 · Year 11

The inverse square law for light with a phone light meter (syllabus practical)

Module 3: Waves and Thermodynamics (Ray Model of Light)

Practical, model not builtLow risk

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The idea

Light from a small source spreads over an expanding sphere, so illuminance on a surface falls with the square of the distance.

What you need

Small bare filament lamp on a 6 V supply, or a single LED in series with a 330 ohm resistor on the same supply (about 10 mA; an LED connected straight across 6 V draws too much current and fails), as a small source
Phone light-meter app (phyphox or Physics Toolbox) or a lux meter, metre rule, dark room
Alternative geometric method from the NSW Department of Education Module 3 guide (Activity 8b): card with a 1 cm by 1 cm window, graph paper

How to do it

Darken the room; place the lamp at one end of the rule and the phone sensor facing it at 0.25 m; record the lux reading.
Move to 0.35, 0.50, 0.71, 1.00 and 1.41 m; take three readings at each.
Subtract the background reading taken with the lamp off.
Plot illuminance against 1/r^2 and check for a straight line through the origin.
Geometric version: keep the bulb 10 cm behind the 1 cm by 1 cm window, set the window card 4, 5, 8 and 10 cm from the graph paper and count the lit squares; the lit area grows with the square of the bulb-to-paper distance, from 1.96 cm^2 at 14 cm to 4.00 cm^2 at 20 cm.

What you should see

Doubling the distance reduces the reading to a quarter: a lamp giving 400 lux at 0.25 m gives about 100 lux at 0.50 m and 25 lux at 1.00 m. The illuminance against 1/r^2 plot is linear; departures at short range come from the lamp not being a point and at long range from reflected light in the room.

What changes

What you change: distance from the lamp
What you measure: illuminance
What you keep the same: lamp brightness
sensor orientation
background light

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Light gets weaker because it wears out with distance; the same energy is shared over a larger area.
Moving twice as far halves the brightness; it quarters it.

Safety card

Low riskLearners carry it out

Hazards

hot lamp
dark room trip hazard

Controls

LED source where possible
clear floor before darkening

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH11-10 PH11/12-4
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-02 PY-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities