The light could be guided by i) refraction (using lenses), ii)reflection and iii) total internal reflection. Guidance by refraction and reflection is lossy and hence is limited only to shorter distances. Guidance by total internal reflection is most often used as the losses are only due to absorption in the medium.
A common example is the propagation of light in an optical fiber
which in its simplest form consists of a circular core of uniform
refractive index surrounded by a cladding of slightly lower
refractive index. The light is launched into the entrance face of
the fiber. The light is propagated by the total internal reflection
at the interface between core and cladding. However the rays
incident at angles larger than a certain angle, called the cut-off
angle, suffer both refraction and reflection at the interface
between the core and the cladding. They, therefore, are not guided.
Due to this the optical fiber has a numerical aperture. The
numerical aperture is given by the square root of
(n Consider the figure which is a section of the optical fiber. The
refractive indices of the core and the cladding are n The angle of incidence at the entrance face for which the ray strikes
the core-cladding interface at the critical angle is called the cut-off
angle. The ray is guided for all the angles of incidence smaller than
the cut-off angle at the entrance face. Also the numerical aperture is
equal to n | |

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

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