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

Infra-red and ultraviolet radiation


This experiment shows the overlap of infra-red radiation, visible light and ultraviolet radiation, all produced by the same source.

Apparatus and materials

parallel beam projector

low voltage power supply, variable

high dispersion prism


cell holder with one U2 cell (1.5 V)

milliammeter (1 mA)

infra-red and ultra-violet filters

printing paper (P153) developer, fixer
fluorescent paper (green)

screen of non-fluorescent white paper and support

4mm leads

Health & Safety and Technical notes

Read our standard health & safety guidance

The parallel beam projector and prism are used, as shown below, to cast a spectrum onto a screen made of a piece of board about 0.3 x 0.2 m to which is pinned a sheet of non-fluorescent white paper set up about 0.5 m from the projector. As usual, adjust the lamp until its image would be in focus on the screen, and rotate the prism to the position of minimum deviation. The white paper, which could be blotting paper, can be tested for lack of fluorescence with an ultra-violet lamp and the ultra-violet filter, in a dark room.

For demonstration purposes, the lamp may be overrun by up to 30 per cent. It should have a linear filament.



Spectrum demonstration

a Connect the phototransistor to the U2 cell and milliammeter in series, and put it just in front of the screen. If the projector is rotated to sweep the spectrum across the phototransistor, a peak response will be found beyond the visible red region.
With the transistor in the region beyond the peak, the effect of the filters can be shown.

Do either b or c. Step c is quicker, but shows less of the ultra-violet.
b Pin a strip of daylight printing paper to the screen and expose it to the spectrum for several seconds, marking the limit of the visible blue-violet with a soft pencil. Subdued incandescent room lights may be left on. Develop the paper in front of the class, when it will be seen that the paper is blackened well beyond the visible region. The dyes in the paper make it insensitive to parts of the visible spectrum.

c Pin the strip of fluorescent paper so that the lower half of the spectrum falls on it, the upper half still falling on the white paper. In a darkened room, some fluorescence can be seen beyond the visible if the lamp is overrun. Much of the fluorescence is in the visible blue-violet, but the difference is shown up by use of the ultra-violet filter to remove much of the visible region.

Teaching notes

1 This experiment lends some support to the 'family' view of electromagnetic radiation is the degree of overlap in properties shown by different parts of the spectrum.

For example, long wave radiation from a hot object can warm things up, and so can the radiation with wavelength of the order of centimetres or millimetres used in modern radar, and indeed also in microwave ovens. But these radio waves in turn share many properties with other radio waves of much longer wavelength.

2 A phototransistor will detect radiation in a spectrum from a lamp across the visible and well beyond the red, in the infra-red region where the most noticeable property is the warming up of an object held in the radiation.

Photographic paper is affected over much of the visible spectrum, but also well beyond the blue, in the ultra-violet region where another noticeable effect is the fluorescing of certain paints.

This experiment has yet to undergo a health and safety check.

Related experiments

Introducing waves

Red-hot heater and curved mirror

Beyond the visible spectrum