For centuries the rules of optics were simple; light makes a single bend when it moves from one medium to another. Those rules have now changed, and the result is a flat funhouse mirror of phase discontinuity.

Light moves in a straight line, and at a certain speed, through a regular medium. Depending on the density of the medium in question, it moves faster or slower. In the vacuum of space, it moves at its maximum speed. When it hits earth's atmosphere, it slows down a bit. When moving through a patch of water, it moves even more slowly. This slowing of light leads to a behavior that most people have seen since they were kids. Light bends when it hits a new medium at an angle, leading to the distortion that people see in swimming pools or glasses of water.

The principle of bending light is fairly simple. Imagine being on roller skates and towed along over pavement. Now imagine being towed towards rougher ground. If both feet hit that ground at the same time, you'll slow down, but you won't change course. On the other hand, if your left foot hits the rough ground first, it will slow while the right foot keeps moving, and you'll swing around to the left. If you are towed to smoother ground, and the left foot hits first, the left foot will accelerate on the smooth surface and you'll swing to the right. This is like what happens to light as it moves from one medium to another. Know enough about the two media, and the angle of the light, and it was possible to predict which way light will bend, and how much.