# Refraction and Snell`s law ## Snell’s law

Refraction is a phenomenon where the light path is bend when it passes from one medium to another. Snell`s law explain refraction accurately.

So, what causes the refraction to occur?  The refraction of light takes place when light travels from a dense medium (one with high refractive index) to another with lower density ( lower refractive index). In other words, refraction occurs because of change of light speed when it enters a second medium.

It is known by other two names; the law of refraction or Snell-Descartes law.  Snell`s law is the ratio of sine of the angle of incident to sine of angle of refraction when light pass through two medium. This is simplified by the formula below;

Where i is the incident angle and r is the angle of reflection.   The Snell’s can also be describes using the ratio of velocity of light for the two media in question. It can be simplified as follows;

Where vi is the velocity of light in the medium on which the light is incident to and vr is velocity of light in the second medium.

When comparing two media, the Snell’s law is the ratio of the two refractive indices of the media of interest. It is simplified as follows;

Where ni is the refractive index of the medium on which the light is incident and nr is the refractive index of the medium in which the light is refracted through.  The three can be combined as follows;

## Total internal reflection and critical angle

It is possible for the light to bend until it is internally reflected within the first medium (when light is traveling from medium with large refractive index to a medium with a smaller refractive index). This is what is known as total internal reflection.  The phenomenon takes place as shown in the figure below.