Evidence for Electromagnetic Waves You claim that light, microwaves, and X-rays are all the same kind of stuff, but they seem very different to me. How do we know that these are all just electromagnetic waves with different wavelengths? One thing that all the forms of electromagnetic radiation have in common is that they can travel through empty space. This is not true of other kinds of waves; sound waves, for example, need some kind of material, like air or water, in which to move. So how do we know that all the different kinds of electromagnetic waves can travel through empty space? Well, it's obvious that visible light can do this; that's why we can see light from the sun and stars, which are separated from us by enormous stretches of emptiness. Astronomers have been making observations of this visible light for centuries. Modern astronomy, though, has other ways of observing the cosmos; instruments have been developed that can detect types of radiation other than light, so that we can now take "pictures" of the sky in infrared, radio, or x-ray wavelengths. So all those kinds of waves must be able to travel through outer space, just the way light can. Exactly! The fact that this information can reach us, across millions of miles of space, points to a fundamental similarity between light and these other forms of radiation. They all must have some method of moving through empty space, boldly going where no other waves have gone before. (We'll discuss the way that electromagnetic waves move later in this section.) And not only can they all travel through the void, they all do it at the same speed: the speed of light. If they moved at different speeds, the planets, stars, and galaxies would appear to be in different places depending on which wavelength we used to view them--but they don't. So you see, it isn't so incredible to say that light, x-rays, and microwaves are all different forms of the same thing. To see some pictures taken using different kinds of electromagnetic waves, check out this collection of astronomy links. (There is also a text-only version of these links, which loads more quickly.)

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