Physics 2000 Einstein's Legacy Microwave Ovens

Standing Waves

The first thing we need to understand is what happens when a wave bounces off the wall of a microwave oven...

It bounces off?

Sure, it reflects off the metal wall. That's why it is safe to be near a microwave oven while it is on; the metal keeps the microwaves in.

The wave doesn't actually bounce back like a rubber ball. Come with me and I'll explain to you how the wave changes phase when reflecting off a metal surface.

But if the waves bounce back, don't they "interfere" with themselves?

Yup, but that's a useful thing. Take a look at the demonstration below. The two gray waves are moving in opposite directions, as if they are just bouncing back and forth. The red wave is the "sum" of the two gray waves as they interfere with each other.

Cool...the red wave doesn't look like it is moving anywhere, it is just moving up and down. And if I change the wavelength of the gray waves, the red wave changes with them.

Right. We call this a "standing wave" because it doesn't seem to move. The points on the red wave that don't even move up and down are called "nodes" and those are the cold spots in your microwave.

Why is the red wave taller than the gray waves?

Remember, we're adding the waves together. In fact, we now have to look at what happens to the red wave as the gray wave bounces back and forth not just once but many, many times...

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