Computer Graphics and Human Computer Interaction group

Refraction of Light

Consider an interface between two isotropic (having same optical properties in all directions) and homogeneous media of refractive indices n1 and n2 respectively. Consider a ray in the medium of refractive index n1 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 n2 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:

i) The incident ray, the normal and the refracted ray lie in the same plane.
This plane is called the plane of incidence.

ii) The angle of refraction r is obtained from the relation

n1 sin i = n2 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 n2=-n1 is substituted in the law of refraction. It, therefore, also has two parts.

The following two cases are of interest:

  1. When n1<n2
    In this case the angle of refraction is always smaller than the angle of incidence. Therefore there will always be refraction as the angle of incidence is increased until 90o.

  2. When n1>n2
    The angle of refraction is always greater than the angle of incidence. As the angle of incidence increases, the angle of refraction increases a bit faster. It would therefore reach a value of 90o for some value of angle of incidence known as the critical angle,ic. For angles of incidence greater than the critical angle, there is no refraction. Instead, the ray is totally reflected. This phenomenon is known as total internal reflection and plays a very important role in light guidance.





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


Main Page/ Light Guidance

© Copyright, 1998 Dr.Vijayalaxmi Sirohi, Computer Center, Indian Institute of Technology Madras,
Chennai 600036, India
and
Prof.Dr.Peter Gorny, Informatics department, University of Oldenburg, D-26111 Oldenburg, Germany
Designed, developed and coded by Dr.Vijayalaxmi Sirohi
Technical support for optics provided by Prof.Dr.R.S.Sirohi, Physics Depaartment, Indian Institute of Technology Madras,
Chennai 600036, India