Optics: Refractive Index, Part 1

The refractive index of materials is one of the most obviously useful optical phenomena I can think of. We've been taking advantage of it for thousands of years, because that's how long we've used lens of some sort. The refractive index is why we have binoculars, telescopes, microscopes, eyeglasses, and why people who have really bad eye sight, like me, don't have to wear coke bottle lenses any more. Its what makes a straw in a glass of water look like its bent and that fish you are trying to catch ninja-style look elsewhere than it is. The fact that it is slightly different for different colors of light lets us use prisms to make rainbows.

High refractive index dispersive lens

So, what is this thing and how does it work?

In optics-speak, the refractive index is the reaction of bulk material to oscillating electromagnetic fields. Which is basically a science-y way of saying 'this is how we observe it reacting to light' or "how light travels through this substance". We measure the refractive index of a material with a dimensionless number n, and if you look at optics equipment it will usually specify at what wavelength it has that value.

You can think of it kind of like a clothing size. If you are a woman, you are used to buying clothes in size 8 or 14 or 0. Eight what? No one knows, but you know your size, and you know your size at store A is an 8 and at store B is a 10. Similarly BK7 or "Schott" glass has a refractive index of 1.53 for blue light and 1.51 for red light¹

Why does light travel differently through different materials? It all comes down to atoms and how those atoms are arranged. All atoms have a positive, heavy nucleus made of protons and neutrons and a negative cloud-shell of much lighter electrons. Electrons are repelled by positive electric fields, while protons are attracted. However, since the nucleus is about 1000 times heavier than the cloud-shell, we usually just think about the electrons moving, and assume the nucleus stays in place. Light is just oscillating electric and magnetic fields at a specific range of frequencies. At other frequencies, we call these waves microwaves, radio waves, x-rays and dozens of other things. So when the electric field part of the light encounters an electron cloud, its going to cause that cloud to shift, inducing a dipole moment.

Not to scale

As the electron gets pushed back and forth by the incoming electric field, it begins to radiate at the same frequency, but slightly delayed in phase. In other words, its out of sync with the first wave.

In an object you can hold, say a magnifying glass, there are trillions and trillions of atoms doing this, and the waves that one atom makes affects another. How the electrons move begins to be limited by the molecular structure of the substance they are forming, any kind of crystalline structure, impurities. The interaction of the original wave with all these new waves results in what we, in the macroscopic world, observe and call the refractive index.

In part two, we'll look at some of the cool effects this gives us, including magnification, and 'bending' chopsticks in water.

~PhysicsGal