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)
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.
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.
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