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
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.
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.
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