There have been a few methods described in this journal and elsewhere for measuring the wavelength of the standing electromagnetic waves in a microwave oven. Typically, these involve melting chocolate, cheese, or some other substance on a plate that is prevented from rotating. In this article I describe a more dynamic and colorful technique that utilizes a temperature-sensitive liquid crystal sheet. With this method one can not only measure the wavelength of the microwaves, but also calculate the speed of these waves, which intriguingly turns out to be the speed of light. It also has the advantage of looking more like a wave than the other methods.
References
1.
Alistair
Steyn-Ross
and Alister
Riddell
, “Standing waves in a microwave oven
,” Phys. Teach.
28
, 474
(Oct.
1990
).2.
Bill
Hammack
explains and demonstrates how microwaves work and uses cheese to measure wavelength: “How a Microwave Oven Works,” https://www.youtube.com/watch?v=kp33ZprO0Ck.3.
I discovered that this technique was previously albeit briefly discussed in a 1990 letter from
Barbara
Andereck
, Phys. Teach.
28
, 580
(Dec.
1990
).4.
For a general description of microwaves, see
Heather
Hosack
, Nathan
Marler
, and Dan
MacIsaac
, “Microwave mischief and madness
,” Phys. Teach.
40
, 264
(May
2002
).5.
The electromagnetic waves in a microwave oven are actually 3D standing waves (rather than traveling waves) and must be solutions to Maxwell’s wave equation with boundary conditions for an oven of given dimensions, which results in a more complicated formula for the speed of light in terms of the wave number, but the approximation above gives a surprisingly satisfying result. See
S.
Kamol
, P.
Limsuwan
, and W.
Onreabroy
, “Three-dimensional standing waves in a microwave oven
,” Am. J. Phys.
78
, 492
(2010
).6.
This type of experiment was performed using an infrared camera:
Jouni
Viiri
, “Temperature distribution in a microwave oven
,” Phys. Teach.
36
, 48
(Jan.
1998
).7.
As another way to demonstrate the three-dimensional pattern, but with small neon bulbs, see “Neon Bulbs in a Microwave,” https://www.youtube.com/watch?v=FzKwxqV0QJ8.
© 2019 American Association of Physics Teachers.
2019
American Association of Physics Teachers
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