Snell's law was one of the first physical laws that I learned as a young, fresh faced undergraduate. OH, how time has made me jaded. It’s actually a very simple law that has to do with the behavior of light refraction through various mediums. Let’s explore this law, shall we.
You know how when you stick something like a straw into a glass of water, the straw looks kinda bent? You’re seeing Snell’s law in action. But before we talk about the law itself, we have to make sure everybody is on board with the definition of a “normal”. The normal is just an imaginary line perpendicular to any surface. Perfectly perpendicular at a 90 degree angle to the surface. Easy enough. The only tricky part comes in when you’re dealing with a curved surface like a lens.You can see from the diagram that the normal depends on where you are on the surface of the lens. We’ll come back to this in just a second because this is why lenses are so good at doing what they do.
Back to Snell’s law. Snell’s law says that if a beam of light moves from a medium of high velocity (like air) into lower velocity (like water or glass) the beam is going to be refracted TOWARDS the normal. We can see this in the picture below:
You can see that instead of traveling along its original path, the beam bent up towards the normal. The opposite happens when you move from a medium where the beam has a low velocity to one where the beam has a higher velocity, like moving from glass to air. In this scenario the beam moves AWAY from the normal. Now let’s check out what happens in a lens:
Do this to a lot of beams of light and you’ll be able to focus those beams onto a single point!
Ta-Da! Now that you understand the principles of Snell's law, you can feel good about being a juvenile misfit while you're burning a line of ants with a magnifying glass.