L3-16. 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

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.