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

Transmitting and absorbing radiant energy

Class practical

Simple observations on transmitting and absorbing radiant energy.

Apparatus and materials

Radiant heater, connected to the mains

Insulating sheet approximately 250 x 250 mm with hole

Glass plate approximately 250 x 250 mm

Retort stand, boss, and clamp

Slab of rock salt, approximately 250 x 250 mm

Health & Safety and Technical notes

Supervise this experiment closely. Watch that students do not get too close to the heater.


a Set up the heater on the bench and connect it to the a.c. mains. Fix the insulating sheet in front of the heater with a retort stand, boss, and clamp. Adjust it so that the hole is level with the glowing element.
b Ask students to look at the element through the hole in the insulating sheet. They should hold the back of their hand near the hole to feel the radiation.
c Students should now use their cheek as a detector, about 25 cm away from the hole. Insert a book between cheek and hole to see what happens; move the book in and out several times.
d Move farther away from hole until the heating effect can only just be detected on the cheek. Repeat the experiment of inserting a book. The book can be put in by a partner either near the cheek or near the source. See if there is any difference due to the time taken for the radiation to travel.
e Insert the glass plate between the cheek and the hole. Move the plate in and out several times. This can be repeated using two sheets of the glass plate held together to give a thicker sheet. (Be ready to move out of the way quickly if it is too hot when the glass plate is removed.)
f If a slab of rock salt is available, replace the glass plates with the rock salt.


Teaching notes

1 In these experiments, special detecting instruments such as thermopiles are deliberately avoided. Students use their cheeks or the backs of their hands as the detecting device.
2 Students should notice that the radiation appears to travel in straight lines through the hole. As they move their head from side to side, the heating effect is cut off and so is the light. Both heating effect and the light appear to travel together. Also the intensity of the radiation varies with distance from the source, and the radiation does not pass through thick materials such as a book. The radiation appears to travel instantaneously when the heater is aligned with the hole because it has a high speed.
3 Although visible light gets through the glass plate, the more abundant longer wavelength infra-red waves do not appear to pass through the glass. Rock salt is translucent to visible light and transparent to infra-red.
Links can be made to greenhouses and sealed-up cars on a hot day. Radiation from the Sun reaches us across empty Space.
This experiment was safety-checked in April 2006