1. Refraction Why does this happen?
Light waves travel in straight lines through empty space, but more interesting things happen when they move from one material to another. We do the same thing ourselves.
Have you noticed how your body slows down when you try to walk through water? You go racing down the beach at top speed but, as soon as you hit the sea, you slow right down. The dense (thick) liquid is harder to push out of the way, so it slows you down. Exactly the same thing happens to light if you shine it into water, glass, or plastic: it slows down. This makes light waves bend. This is called refraction.
You can see this for yourself by putting a straw in a glass of water.
Why does this happen?
The speed of light is always the same when light travels in a vacuum. But light travels more slowly in some materials. Light slows down when it moves from air to water and it speeds up when it moves from water to air. This is what causes the straw to look bent.
Refraction is amazingly useful. The glass lenses in eyeglasses bend (refract) the light. Bending the light makes it seem to come from nearer or further away. This corrects the problem with your sight. Basically, your eyeglasses fix your vision by slowing down incoming light so it moves direction slightly. Although light normally travels in straight lines, you can make it bend round corners by shooting it down thin glass or pipes called fibre optic cables. Reflection and refraction are at work inside these "light pipes" to make rays of light follow an unusual path.