Science Questions

Can you reflect x-rays or radio waves?

Sun, 14th Jan 2007

Part of the show Naked Science Q & A and the World of Chemistry

Question

Thor Bjarnason via email asked:

Mirrors reflect visible light. But how effective are they at reflecting other electromagnetic waves like x-rays and radio waves?

Answer

It depends what you make the mirror out of. If you're looking at radio waves then the mirror will have to be made of thicker metal, because as you increase the wavelength you also have to increase the thickness of the metal to get the same reflectivity. That's actually how satellite dishes work. They're basically a big curved mirror that concentrates all of the microwaves coming down from the satellite. They're often full of holes to keep the weight down, and this doesn't matter because the wavelength of the waves is larger than the holes. This is the same principle as seeing a light on in your microwave. You can see the light escaping through the door but the microwaves aren't escaping because they're too long. Once you get beyond visible light into the shorter wavelengths; ultraviolet light is easy to make mirrors for, but x-rays are very difficult. And so making x-ray telescopes is very difficult. Sometimes they do it by using a bag of gas to act like a lens rather than mirrors or by using a metal mirror but at a very grazing angle which makes the mirror very large. Mobile phone waves are at the microwave end of radio waves, and a sheet of aluminium would work nicely as a mirror for those.

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