Further experiments on radiation

Class practical

Shows the properties of infra-red radiation.

Apparatus and Materials

For each group of students

• Mains lamps and holders, gas-filled and vacuum-filled (60 watt). Please note this piece of apparatus is very difficult to find.
• Copper calorimeter, large or a steam chest
• Immersion heater (mains powered)
• Vegetable black
• Thermometer (0°-100°C)
• Paper, white

Health & Safety and Technical Notes

If mains lamps are used, the holders should be the safety pattern where the contacts are isolated when the lamp is removed.

Read our standard health & safety guidance

Pre-focus (P13.5s) torch bulbs (2.37W) are available both vacuum and krypton filled. Distinguishing between these will require a sensitive detector.

Procedure

1. Keep some water boiling inside a copper box using an immersion heater. Alternatively the box can be kept at 100'C by passing steam through it. One face is shiny; one face is dull black having been coated with vegetable black, one face is covered with white paper. Use the back of the hand to compare the radiation.
2. Put a thermometer (0°-100°C) in a metal container filled with boiling water, and observe the rate of cooling. Do this first with a well-polished container. Then with a layer of vegetable black painted on the outside.
3. Switch on a 60 watt gas-filled mains lamp and a 60 watt vacuum-filled mains lamp near each other. Ask students to decide, as a detective problem, which of the two has gas inside.
4. Put a cheek near a mains lamp and switch it on and off to feel how promptly the radiation reaches the face.

Teaching Notes

• Step 1 is a version of Leslie's cube and demonstrates the differing amounts of radiation emitted from differently coloured surfaces.
• In step 2, the matt black can cools down most quickly because more radiation is emitted from it. Cooling curves could be plotted.
• In step 3 the surface of the gas-filled lamp will be hotter. This is because of the energy transferred through the gas by conduction, although the energy transferred by infra-red radiation will be similar.
• In step 4 the time lag is too short to distinguish because the radiation travels at a very high speed. (The speed of light.)

This experiment was safety-tested in April 2006