This is an interesting question, one I have often wondered about.Light is a complicated phenomenon. As far as I know, we don't know why light travels so fast. However, we do understand something about what light is. Light is closely related to electricity and magnetism, so let's first talk a little about that. It will turn out that studying electricity and magnetism will tell us almost everything that we need to know about light.
You are probably already familiar with the effects of magnetic fields: magnetic fields are what make magnets stick to refrigerators, and it is the earth's magnetic field which makes a compass point to the north. Electric fields are a little different: when you turn on a flashlight, it is the electric field in the batteries which makes electrical current flow through the wires and through the light bulb, thus lighting up the bulb. Electric fields are also what makes opposite charges attract each other and like charges repel each other. Can you think of other effects of electric and magnetic fields?
When you carefully study the behavior of electric and magnetic fields, you find that it is possible for waves of electric and magnetic fields to travel through empty space. What do I mean by "waves"? You've probably tried holding one end of a rope or string while a someone else holds onto the other end. When you wiggle your end up and down, you see waves travel from your end of the rope down to the other end. (Here's a question for you:
How fast do the waves travel down the rope? Does it depend on how fast you wiggle your end back and forth?)
When I say "waves of electric and magnetic fields," I'm talking about something similar to the waves in the rope. Similar to wiggling the end of the rope, we can change the electric or magnetic field at one point, and that disturbance travels away from us. (One way to change the magnetic field at a point is to hold a magnet there, then wiggle it back and forth.) Unlike the wave in the rope, these waves actually travel away from us in all directions. It turns out that in a vacuum, for example in outer space, these waves travel at exactly the speed of light! It doesn't matter how rapidly the fields wiggle back and forth, they will always travel at the same speed. (In air, the waves go ever so slightly slower than in vacuum, and in glass, they can go only about two-thirds as fast as they would in vacuum, but this is not important right now. From now on, when I talk about the speed of light, I mean the speed of light in a vacuum, and I'm talking about waves traveling in a vacuum.) The conclusion that physicists have drawn from this result is that light consists of waves of electric and magnetic fields. This is a fascinating result: that we learn something about light by studying magnets and electrical currents.
You may wonder why we can't see the waviness of light. Well, the fields that make up the light that we can see are wiggling very, very fast - about 500,000,000,000,000 times every second. (So unfortunately, you can't wiggle a magnet back and forth by hand fast enough to see any light come out!) Your eye just can't tell that the fields are actually wiggling. All it can see is a steady amount of light.
Speaking of how fast the fields are wiggling, they can wiggle at different rates. When the fields are wiggling about 500,000,000,000,000 times every second, your eye sees red light. If your eye looks at fields which are wiggling about twice that fast, or about 1,000,000,000,000,000 every second, the light looks blue. All of the colors of the rainbow are due to the fields wiggling at slightly different rates.
Visible light is not the only phenomenon due to wiggling electric and magnetic fields. The reason you hear music from a radio station is that your radio can detect electric and magnetic fields traveling through the air which are only wiggling about 100,000,000 times every second. If you tune to 93.1 FM, for example, your radio is picking up fields which are wiggling 93,100,000 times every second. For 101.9 FM, the fields are wiggling 101,900,000 times every second. (The rate of wiggling is called the "frequency." What range of frequencies can an FM radio tune in to?) This is nowhere near as fast as the wiggling in visible light. But radio waves and visible light are very similar - they are both waves of electric and magnetic fields, which just happen to be wiggling at different rates. Therefore, they both travel at the speed of light. Visible light, ultraviolet light, infrared radiation, radio waves, X-rays, gamma rays, microwaves, and radar waves are all made up of electric and magnetic fields wiggling at different rates, and they all travel at the speed of light.
Now, I mentioned earlier that light is a complicated phenomenon. Long after the electromagnetic wave theory of light was accepted, it was discovered that it could not quite explain every
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