 I want you to observe what happens when light travels through a different medium. I'm going to place this lapel pin in the bottom of a cup and over near the side. What you're going to do is you're going to observe what happens as I pour water into that cup. Now what happens is light strikes the surface and it's going at the speed of light in air, but when it hits the water it slows down and as it does it bends. And that's what allows you to see the pin in the bottom that you couldn't see before. It's because the light is bent in the water.