Refraction of light: Introduction

Check this out! I'm gonna make light turn! You think I'm stupid? I know that light always travels in a straight line. Turn the light off, and you'll see!

Bottle of water. Flashlight. Look here, when I make a hole in the bottle. The water stream is like a cable transporting the light, making it turn! Wooah.

Light turning! But shouldn't that be impossible? Yes, as Philip says, it is true that light always travels along a straight line. But the experiment still works -- the light follows the stream of water, and turns. Let's take it from the beginning...

Light travels in a straight line, really really fast. In empty space -- vacuum -- light travels at almost three hundred thousand kilometers per second. But when light is traveling through a transparent material instead of a vacuum, it moves slower. Through air, light travels almost as fast as in a vacuum. But through glass, or water, a ray of light moves much slower.

This property -- that light travels at different speeds through different media -- makes it capable of changing direction when it changes from one medium to another. A change in speed that causes a change in direction? It's not as strange as it sounds. Look here: Here's a car on its way to the beach. On the tarmac, the tyres have a good traction, and the car moves fast.

If the car drives on the sand, however, the tyres skid, and the car moves slower. Look now, when the car enters the sand diagonally from the tarmac. The left side wheels of the car, at the top on the screen, are still on the tarmac, while the right side wheels are on the sand. The side of the car that's on the tarmac is now slightly faster than the the other side. And what happens if one side travels faster than the other?

The car turns! Now, all four wheels are on the sand, only slower than before. and the car is going straight ahead, Imagine now that the tarmac is air, where light travels almost as fast as in vacuum. And the sand is glass, where light travels slower. And the car is a ray of light.

When the ray of light enters the glass, it slows down on one side before the other, and the ray changes direction. When the ray of light exits the glass and re-enters the air, the same thing happens, but in reverse. One side enters the air and speeds up before the other, and the ray of light is now traveling in the same direction as when it entered the glass. When light changes direction like this, we say that it is refracted. Exactly how much the ray of light is refracted depends on the ratio between the speed of light in the two materials.