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

Investigating the spectrum with an electronic detector


A class demonstration of the spectrum.

Apparatus and materials

Compact light source (quartz iodine lamp)

Power supply, low voltage, variable

Convex lens, large

High-dispersion prism (flint or lead glass)

Screen, white



Demonstration meter (2.5-0-2.5) mA

Health & Safety and Technical notes

Treat the compact light source with respect as it is a significant source of UV; do not look directly at the bulb. The lens will effectively filter out the UV.

A suitable 63 mm diameter +6D plano-convex lens is obtainable from Knight Optical UK Ltd, part no. LPV16063

Connect the phototransistor in series with a 1.5-volt cell, and the demonstration meter with a 2.5-0-2.5 mA scale. A slit may be needed to place over the phototransistor, through which the radiation can pass. 
The phototransistor will probably be mounted on a board with a red and a black terminal. The red terminal should be connected to the positive side of the cell. 
Note that no slit is needed if the source is compact or if it is a line filament, parallel to the prism's edge. 

Apparatus set-up



a Position the convex lens 20 cm in front of the lamp. If a plano-convex lens is used, face the plane surface towards the lamp. An image of the hot filament will form far away. Put the high-dispersion prism after the lens and near to it. Position the white screen so that it is at the image distance from the lens, about 3 m away. The spectrum will be seen. 

b Move the phototransistor through the visible spectrum and show the response on the meter. It will be clear that there is radiation beyond the visible end of the spectrum. 

Teaching notes

1 This demonstration shows the variation in intensity of the different colours of light from a hot filament in a glass envelope. 

2 The radiation falls outside the visible region of the spectrum, indicating infra-red and ultra-violet radiation. But because the prism and the lens are made of glass, there is a sharp cut-off in the infra-red and little if any response in the ultra-violet. 
3 You could extend the demonstration using a small blackened thermistor, infra-red or ultra-violet sensor, although the restriction placed by the absorption of those wavelengths by glass remains. 

This experiment was safety-checked in January 2007


Related guidance

The electromagnetic spectrum

Learning from spectra



Knights Optical UK Ltd


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