Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Investigating light intensity from a lamp


Measuring how the light intensity of a lamp varies with input power.


Apparatus and materials

Power supply, LV

Lamp, 12 V 24 W

Lamp holder (SBC for LV use)

Rheostat (10 - 15 ohms)

Demonstration meter (0 - 15 volts DC)

Demonstration meter, 0 - 5 amp DC

Light meter (or photographic exposure meter)


Health & Safety and Technical notes

Read our standard health & safety guidance

Digital light meters suitable for education use can be obtained from several electrical equipment suppliers for £30 or so. For example, try Maplin.





a Set up the circuit shown with the light meter some 10 - 15 cm from the lamp.

A circuit

b Record values from the light meter, together with ammeter and voltmeter readings for input powers ranging from 10 to 30 watts (corresponding to a change of 7-14 volts).
c Plot a graph of light intensity against power input.


Teaching notes

1 The graph of light meter readings against input power is a very important one when the efficiency of lamps is being investigated. This graph should give approximately a straight line, not passing through the origin.

2 The light meter may need to be placed further from the lamp. For a 100-watt lamp, meters will be needed recording up to 250 volts AC and 500 mA AC.
3 Energy efficient lamps could also be tried but if high voltages are needed, then suitable precautions need to be taken.
This experiment was safety-checked in January 2006


Related guidance

Power and energy

Helpful language for energy talk