Snell's law is a formula that describes the relationship between the angle of incidence and the angle of refraction when a ray passes through the boundary between two different mediums. Only some of the light is reflected the intensity of this reflected light is modelled by Fresnel's equations.

The law for light is defined as:

• Definitions
  • is the index of refraction of the medium that the light is exiting
  • is the angle of incidence from the line normal to the boundary
  • is the index of refraction of the medium that the light is entering
  • is the angle of refraction from the line normal to the boundary

Total Internal Reflection

If there is no value that satisfies the equation

then this means that all light energy transmitted through the medium will be refracted back into the first medium. This is called total internal reflection.

Critical Angle

The angle for which total internal reflection occurs is known as the critical angle, any angle greater than the critical angle will be totally internally reflected. It can be calculated by taking the inverse sine of the ratio of the indexes of refraction:

Examples

All of the light is totally internally reflected back into the glass