Consider an interface between two isotropic (having same optical properties in all directions) and homogeneous media of refractive indices n_{1} and n_{2} respectively. Consider a ray in the medium of refractive index n_{1} which strikes the interface at point P. Draw a normal to the interface at the point P. The angle between the incident ray and the normal to the interface is the angle of incidence,i. Due to refraction the direction of the ray, called the refracted ray, in the medium of refractive index n_{2} is deviated. The angle between the refracted ray and the normal to the interface at P is the angle of refraction, r. The direction of the refracted ray is governed by the law of refraction, which is stated in the following two parts:
$n$_{1} sin i = n_{2} sin r In fact when a ray travels from one medium to the other, bounded by parallel plane, n sin x is an invariant. The law of refraction is also known as the Snell's law. The law of reflection is obtained when n_{2}=n_{1} is substituted in the law of refraction. It, therefore, also has two parts. The following two cases are of interest:


The light path under varying conditions of input parameters can be studied by the following: 