# Further experiments on radiation

##### Class practical

Shows the properties of radiant energy.

#### 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.

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

#### Procedure

a 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.

b 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.

c 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.

d Put a cheek near a mains lamp and switch it on and off to feel how promptly the radiation reaches the face.

#### Teaching notes

1 Step a is a version of Leslie's cube and demonstrates the differing amounts of radiation emitted from differently coloured surfaces.

2 In step b, the matt black can cools down most quickly because more radiation is emitted from it. Cooling curves could be plotted.

3 In step c the surface of the gas-filled lamp will be hotter. This is because of the energy conducting through the gas, although the energy radiated will be similar.

4 In step d 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-checked in April 2006