[EDIT] This post was migrated from my blog from 2011…
This all below is stuff that kids study in school. :)
Below is a visualization of the behavior of a ray of light as it hits a dielectric interface.
Some key phenomena which show up in the video are:
- The Fresnel term (reflection vs. refraction).
- The Index of Refraction.
- The critical angle.
- Total Internal Reflection (TIR).
- As nd increases the Index of Refraction becomes higher, and so does the Fresnel term, which defines the proportion between reflected and refracted light. The critical angle becomes higher too, so there is more Total Internal Reflection.
When a ray of light hits an interface (assuming an ideal surface), all incident light must be either reflected or refracted. The Fresnel term (controlled by nd) tells how much light is reflected at a given incident angle. All the light that is not reflected is refracted, so both amounts (reflection and refraction) always add up to the total amount of incident light.
The Fresnel term approaches 1 at grazing angles (all light is reflected and nothing is refracted, regardless of nd) and is low (the lower the smaller the nd) at perpendicular angles (more light is refracted).
As a rule of thumb:
- The lower the nd, the lower the IOR, and the more transparent the surface (more glass/liquid-like).
- The higher the nd, the higher the IOR, and the more reflective the surface (more metallic/mirror-like).
- Void: nd=1.
- Air: nd=1.1.
- Water: nd=1.33.
- Glass: nd=1.51.
- Diamond: nd=2.5.
- Metals: nd=20+. (Simplification that ignores Complex IOR).
- Ideal mirror: nd=infinity.
When a ray of light enters a medium with an nd lower than the nd of the previous medium, there is an angle at which the Fresnel term becomes 1 and beyond which light won’t refract anymore. This angle is called critical angle, and beyond it, the surface behaves like a perfect mirror, reflecting back all incident light. This effect is called Total Internal Reflection (TIR).