Energy and Thermal Physics

Investigating light intensity from a lamp

Practical Activity for 14-16 PRACTICAL PHYISCS


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

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.


  1. Set up the circuit shown with the light meter some 10 - 15 cm from the lamp.
  2. Record values from the light meter, together with ammeter and voltmeter readings.
  3. Calculations of the power input (energy transferred electrically per second to the lamp) should range from 10 to 30 watts (corresponding to a change of 7-14 volts).
  4. Plot a graph of light intensity against power input.

Teaching Notes

  • The graph of light meter readings against power input 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.
  • This is an opportunity to discuss the power rating of devices. This is the power input when the device is connected to the potential difference for which it has been designed. It will not transfer energy at the same rate if connected to a lower (or higher) potential difference.
  • The light intensity is a proxy for the power output (the energy transferred per second by the light).
  • 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.
  • Energy efficient lamps could also be tried but if high voltages are needed, then suitable precautions need to be taken.

This experiment was safety-tested in January 2006

appears in the relation P=VI P=I^2R P=V^2/R ΔQ=PΔt
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