Demos: 3D-03 Radiation--Match

A coil-wire heater is placed at the focal point of a spherical mirror. An unlighted match is placed at the focal point of an identical mirror located about a meter away. Upon reflection from the mirror, the rays from the heater are parallel so that when they encounter the other mirror, these rays focus at the focal point. The concentrated radiation is enough to ignite the match.

Directions: Proper positioning of the heater coil and the match is crucial. This is usually done by the demo technician prior to classtime. Just be sure that the apparatus isn’t bumped before you perform the demonstration. Note: This is not a rapid process. It might take a minute or more for the match to light.

Suggestions for Presentation: If you turn on the heater, you can hold your hand the same distance as the match from the heater and barely feel the warmth. Show the students the two mirrors and suggest that mirrors reflect “light” so there shouldn’t be any way to get the “heat” to focus at the other mirror’s focal point. Then, recalling 3D - 1, remind them that light and heat are just names we give different parts of the E&M spectrum. The mirrors should be able to reflect the longer wavelengths as well as the shorter ones.

Applications: Using a reflecting mirror or lens to burn an object.

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