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

Comparing the powers of lamps


The intensity of illumination from different lamps is compared.

Apparatus and materials

Lamp holders (SBC) on base, 2 (Safety pattern batten lamp holders are best)

Mains lamp, 230V 25W, pearl

Mains lamp, 230V 60W, pearl

Screen and post


Health & Safety and Technical notes

Safety pattern lamp holders incorporate a switch so that when the bulb is removed the contacts become 'dead'. The lamp holders must use a double-insulated mains lead fitted with a proper (13 A) mains plug, and a suitable fuse.



a Switch the lamps on and compare the brightness of their illumination.

b Stand the lamps in front of the post so that the shadow of the post falls on the screen. If the lamps are lit one at a time, the apparent intensity of the shadow gives a good comparison of the lamp intensities. 

Apparatus set-up


Teaching notes

1 The two lamps are rated at different wattages. This means that they will transfer energy at different rates. A different number of joules per second are carried from the power supply to the lamp.

2 Students may wish to see the markings on the lamps to confirm the power ratings.
This experiment was safety-checked in February 2006


Related guidance

Power and energy


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