Red-hot heater and curved mirrors

Demonstration

This sequence of demonstration experiments could be used as part of an introduction to waves, in which case students' questions could be answered with further questions.

It could also be used to demonstrate that infra-red radiation reflects in exactly the same way as visible light, or to get a qualitative feeling for its speed.

Apparatus and Materials

• Radiant heater, 300-watt
• Parabolic mirrors, metal-surfaced, 1 pair

Health & Safety and Technical Notes

The old types of radiant heater are not considered sufficiently safe for use in schools. A 300-W safe version is available (see apparatus).

Beware of burns: tell students to stop as soon as they feel anything.

Read our standard health & safety guidance

Procedure

1. Set up the mirrors facing each other, 2 or 3 metres apart.
2. Place the heater at the focus of one of them—using the visible red radiation as a guide.
3. Find the image of the heater formed by the second mirror.

Teaching Notes

• Let students place a hand at the focus of the second mirror, by turns. Avoid telling them what to expect but suggest placing the hand with palm towards the other mirror.
• Say: "Some energy seems to go across from one mirror to the other."
• Ask: "How could energy get across there, so far away?"
• Hold a large sheet of cardboard or plywood as an obstruction just beyond the heater.
• Ask students to try again with a hand at the focus of the second mirror.
• Whip the board away quickly.
• Ask: "What do you feel? Does the supply of energy start again almost at once or is there a delay — as it would if air currents carried the energy?"
• With a student’s hand at the focus of the second mirror, suddenly obstruct the radiation again.
• Ask: "How quickly was the warmth cut off? How fast does the radiation travel?"

This experiment was safety-tested in September 2004