Dielectrics

Clear materials such as water, glass, and diamond are dielectrics. When a light ray hits them, it splits into a reflected ray and a refracted (transmitted) ray. We’ll handle that by randomly choosing between reflection and refraction, only generating one scattered ray per interaction.

As a quick review of terms, a reflected ray hits a surface and then “bounces” off in a new direction.

A refracted ray bends as it transitions from a material's surroundings into the material itself (as with glass or water). This is why a pencil looks bent when partially inserted in water.

The amount that a refracted ray bends is determined by the material's refractive index. Generally, this is a single value that describes how much light bends when entering a material from a vacuum. Glass has a refractive index of something like 1.5–1.7, diamond is around 2.4, and air has a small refractive index of 1.000293.

When a transparent material is embedded in a different transparent material, you can describe the refraction with a relative refraction index: the refractive index of the object's material divided by the refractive index of the surrounding material. For example, if you want to render a glass ball under water, then the glass ball would have an effective refractive index of 1.125. This is given by the refractive index of glass (1.5) divided by the refractive index of water (1.333).

You can find the refractive index of most common materials with a quick internet search.