Evidence for Electromagnetic Waves
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.