In your explanation of why microwaves don’t penetrate the oven door, you said it is because the holes in the screen are smaller than the wavelength of a microwave. Wouldn’t it be the amplitude of the wave and not its wavelength? – P
When a microwave tries to pass through the holes in the metal screen, electric charges in that screen begin to move. The microwave’s electric field fluctuates back and forth rapidly and the charges reverse directions rapidly as a result. If the electric current made up of these charges has enough time to travel all the way around each hole before it reverses directions, it will be as though the screen were made of solid metal and the screen will be able to completely reflect the microwave.
Like any electromagnetic wave, a microwave has a wavelength (the spatial distance between adjacent wave crests) and a period (the temporal spacing between adjacent wave crests). The electric current that a microwave propels through a metal travels about one microwave wavelength during one microwave period. Therefore, the current can work its way around a hole in the metal only if the hole is significantly smaller than the microwave wavelength. The amplitude of the microwave doesn’t matter—increasing the amplitude of the microwave just makes more current flow.