Physics 2000 Einstein's Legacy Microwave Ovens


OK, we just looked at what happens when a wave bounces back on itself to form a standing wave. In this next demonstration let's look at what happens when a wave bounces back and forth a whole bunch of times. (The actual number of waves you get will depend on how fast your computer is.)

OK, why is this so different?

I'll show you. First drag the slider all the way to the right, so that the wavelength is exactly equal to the length of the inside of the box it is bouncing in.

Hey, most of my gray waves disappeared when I did that! Where did they go!

They didn't go anywhere, it's just that they are exactly on top of each other, so you can't tell them apart. When waves bounce back and forth on themselves like this, we call it "resonance".

Oh, I get it. And now if I change the wavelength just a little bit, they start to spread out a little.

And if you change it enough they spread out so much that they start to cancel each other, and the red wave starts to flatten out. In fact, we're only showing a few waves here, but if you were to let the wave bounce back and forth enough times, like a real microwave does in an oven, then it would cancel itself out, at the speed of light, as soon as it began to spread out even a little bit.

Oh, right. We only have a few, slow moving waves here, but a real wave going at the speed of light would be everywhere pretty quickly. Does that mean that the only waves that can bounce around in microwave oven without cancelling themselves have a wavelength as long as the oven itself?

Not exactly. As long as the walls are as far apart as a round number of wavelengths, then the wave will be in resonance. In other words, if the oven can hold two or three waves (or any other round number) then the waves will resonate. But two and a quarter wavelengths, or example, will eventually cancel.

I had some trouble understanding this concept. If you do, too, then come with me to learn how Bathroom Tile can help you understand resonance.

The main thing to understand here is that you can put whatever waves you want to into a microwave. The ones that fit an even number of times will resonate and amplify themselves. The ones that don't fit will quickly cancel themselves out.

This must be why guitar strings play certain notes -- when I pluck the guitar string and only certain vibrations "survive." The others get cancelled.

Yes, exactly. If you look at a guitar string under a strobe light (or even a fluorescent light) you can see it that makes a standing wave. Another experiment you can do is to stand in the shower (they reflect sound well) and start singing while changing the pitch slowly. At certain pitches the sound will suddenly amplify, because the sound waves fit an even number of times between the walls.

You've reached the end of this path. From here you can either go back to the Introduction or read a related page out Microwave Safety

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