Focusing Of Heat Waves By Mirrors

Purpose

To demonstrate that concave mirrors can focus heat waves.

Equipment

Two parabolic concave relfectors, heater on mount, and match on mount.

Images

l3-16

Description

Two parabolic concave mirrors are used to focus heat from a nichrome heater and light a match. ◙◙(1) The heater is first placed about 50 cm from a match, but the heat is not intense enough to light the match. ◙◙(2) Placing one reflector behind the match so that the match is at the focus of the reflector will light the match in 10-15 seconds. ◙◙(3) Turn the heater around so that it is facing away from the match and use the second concave mirror to form a parallel beam of heat, which is then focused onto the match head as in step (2). The match lights in 6-8 seconds!

References

R. E. Berg, Heat-Wave Focusing by Concave Reflectors, TPT 28, 56-57, (1990). ◙List of Material Properties, Machinable Grade A Lava, General Electric Ceramics. ◙Catalog Description, Reflectors for Focusing Radiant Energy, Sargent-Welch. ◙Instructions for No. 1739 Parabolic Reflectors, Welch Scientific. ◙Lecture Demonstrations Data Sheet on Heater Elements.