1. Introduction: The Straw That Bends
Drop a straw into a glass of water. Look at it from the side. The straw looks bent — broken at the surface where air meets water.
You already know why: light bends when it moves from one medium to another. That is refraction.
But here is the question this lesson answers: by how much does it bend? Why does water bend light by one amount and glass by a different amount? What number captures that difference?
That number is called the refractive index. And once you understand what it really measures, you will know why a diamond sparkles more than glass, and why a swimming pool always looks shallower than it actually is.