Refraction And Total Internal Reflection

Stand at the edge of a clear pool and the bottom looks close enough to touch. Step in and the water comes up past your knees, because the floor was never where your eyes put it. Drop a spoon into a glass of squash and the handle appears to snap at the surface. Look along a road on a hot afternoon and the far end of it shimmers. None of those things is happening to the pool, the spoon or the road. All of them are happening to the light on its way to your eye, and all of them are the same single piece of physics.

That piece of physics is this: light does not travel at one fixed speed through everything. It races through empty space and through air, it is noticeably slower through water, and slower still through glass. Every time it crosses from one of those into another its speed changes, and if it arrives at the surface at an angle rather than head on, it has no choice but to swing round into a new direction. That swing is called refraction. In this lesson you will learn what causes it, how to work out which way a ray will turn at any surface you are shown, the one case in which nothing turns at all, and how to run a bench experiment that makes the bending visible on a sheet of paper. By the end you will be able to look at a ray diagram you have never seen before and say confidently which side of the normal the ray comes out on, and why.