Beyond the visible spectrum

Demonstration

This experiment can be used when introducing the electromagnetic spectrum. It shows that there is a form of radiation beyond the visible.

Apparatus and Materials

• Prism, high-dispersion
• Light source, compact
• Positive lens, large
• Screen, white
• Infra-red detector, metered output
• Infra-red detector, metered output

Health & Safety and Technical Notes

Read our standard health & safety guidance

For an infra-red detector, make a potential divider using a photo-diode in series with a 100kΩ resistor, and a 5 V supply across the pair. Connecting a digital voltmeter across the resistor will give the required metered output.

Compact light source: 100 W at 12 volts

The power supply should supply 8 amps.

Procedure

1. Set up the compact light source. The lamp filament should be small enough that no slit is needed.
2. Place the lens about 20 cm from the lamp. (If the lens is plano-convex, its plane face should be towards the lamp.) Move it to make an image of the filament on a white screen, 2 or 3 metres away.
3. Place the prism just beyond the lens and move the screen round to catch the spectrum at the same distance from the lens as before but in the new direction. The spectrum will be pure enough for this demonstration if the prism is turned to minimum deviation. To make the spectrum longer, twist the screen to catch it obliquely.
4. Move the detector across the spectrum near the screen and observe the output readings.

Teaching Notes

The radiation emitted by a hot filament has its maximum not in the visible spectrum, but beyond the red (in the 'infra-red' part of the spectrum). Light is just one (small) part of a family of radiations called the electromagnetic spectrum.

This experiment was safety-tested in February 2006