More of this Feature
•  Part 1:The speed of light
•  Part 3: Joe's Measurements
 Related Resources
•  The Speed of Gravity - Measured?
•  More on Galileo
•  Albert Einstein
•  Light and Electromagnetism  
 
The only equipment you need for this experiment is a microwave, a ruler and chocolate, cheese or any other food that melts. Remove the turntable from the microwave and replace with chocolate on a plate (so the plate does not rotate), and heat until it just starts to melt - about 20 seconds, depending on the power of the oven. There will be some melted hot spots and some cold solid spots in the chocolate. The distance between the hot spots is half the wavelength of the microwaves, and the frequency of the microwaves will often be printed on the back of the oven. The speed of light is equal to the wavelength multiplied by the frequency of an electromagnetic wave (microwaves and visible light are both examples of electromagnetic waves). So from this simple experiment, and some easy math, you can work out the speed of light from Milky Way Magic Stars®!

How it works:
When you turn on your microwave oven, electrical circuits inside start generating microwaves – electromagnetic waves with frequencies around 2.5 Giga Hertz – 2500000000 Hz. These waves bounce back and forth between the walls of the oven, the size of which is chosen so that the peaks and troughs of the reflected waves line up with the incoming waves and form a “standing wave”.
If you pluck a guitar string, you’ll set it vibrating. Usually, you will excite the “first harmonic” – a standing wave that has the string stationary at the bridge and the fret, and vibrating back and forth in the center. With effort, you might be able to excite the second harmonic (try plucking the string in opposite directions 1⁄4 of the way in from either end), then you’ll see the string vibrating back and forth, with the center stationary. This pattern has three nodes or points with no displacement away from rest (there’s a useful mnemonic - a NODe has NO Displacement ): the ends and the center, and two anti-nodes: 1⁄4 and 3⁄4 of the way along its length. There are infinitely many modes, one for each positive integer, with more and more nodes between the fret and the bridge.
As a full wave is shaped like a “sine function” going from zero to a maximum back through zero to a negative maximum and back to zero again. So you can see that the distance between the maximum displacements is one half the wavelength.
The electromagnetic field inside the microwave behaves in roughly the same way – except the vibrations are in “the electromagnetic field”. Where the vibrations are greatest (the anti nodes), you will see the greatest heating, but at the nodes, the chocolate will only melt slowly as heat diffuses into those areas.
Thus, the distance between the melted regions is equal to the distance between the antinodes, and equal to half the wavelength!
 

 
 
 
Next page >Joe Attempts this experiment at home!