Snell's Law.

 

 

The angle of refraction depends on the refractive indices of the two materials in question and at what angle the ray passes between  the boundary of the two. Let us consider the diagram below :

 

 

Snel, or to give him his anglicised name Snell, rediscovered the law of refraction in 1621. This law had been first discovered by Ibn Sahl as long before as 984, but Snell managed to express and publish it as a mathematical function. The first applications were in astronomy, but it is now of critical importance in fiber optics.  Snell died in 1626; his work is yet another example of an idea being useful far beyond the lifetime of its originator.

     Willebrord Snel

    van Royen

 

A diagram to illustrate Snell's law of refraction.

In this case the angle of incidence is the lower, the angle of refraction the upper.

  Snell's law states :      

where n is the refractive index in both cases and    is the angle from the normal.

As the angle of the ray increases when it enters the material having a lower refractive index, a time will eventually come if we move the angle of incidence whereby the angle of refraction reaches 90 degrees and the light refracts along the boundary of the two materials.  The angle of incidence at which this occurs is known as the critical angle.