Herschel’s infra-red experiment

Class practical

This is a recreation of Herschel’s experiment, published in 1800, in which he discovered infra-red radiation.

Apparatus and Materials

For each student group

• Glass prism
• Cardboard box with open top (e.g. photocopier paper box)
• Thermometer, 3
• Marker pen, black
• A4 paper, plain white, one sheet

Health & Safety and Technical Notes

Students should be warned not to look directly at the Sun.

If you use liquid-in-glass thermometers, be careful about breakages. If you use mercury–in-glass thermometers, have a small kit available for clearing up any breakages. Mercury is classed as a hazardous waste and should not be disposed of with normal waste.

Read our standard health & safety guidance

A plastic prism is unsuitable as it is likely to absorb more infra-red radiation than glass.

Procedure

1. Blacken the thermometer bulbs with black marker ink.
2. Cut a notch in the side of the box so that the prism will fit snugly. You need to be able to turn the prism about its long axis while it remains securely in position.
3. Place the sheet of white paper in the bottom of the box.
4. Out-of-doors, position the box with the prism towards the Sun. Adjust the angle of the prism to give the broadest possible spectrum on the sheet of paper. (It may help to place a book under the front edge of the box to tilt it.)
5. Position the three thermometers so that their bulbs are in different regions of the visible spectrum (blue, yellow, red).
6. Note the maximum temperature reached by each thermometer.
7. Repeat, but place one of the thermometers beyond the red end of the spectrum.

Teaching Notes

• In Herschel’s experiment, he placed only one thermometer at a time in the spectrum; the other two were on either side (where no radiation falls), to act as controls.
• Herschel noted that the final temperature reached by his thermometers depended on the size of the bulb. This is because a large bulb has a larger area to absorb radiation, but it also has a larger area to lose radiation, and a greater mass to heat up.

How Science Works Extension: For an approach to this experiment including questions and teaching points, see: William Herschel and the discovery of infra-red radiation

You will find further teaching ideas in the relevant section of the Cool Cosmos website.:

Cool Cosmos

Acknowledgement: This experiment has been adapted from the Cool Cosmos website.