TABLE OF CONTENTS In geometrical optics we consider that light propagates accurately in straight lines, without however, concerning ourselves with the actual nature of light. The basic principles regarding the nature of light were formulated in the latter half of the seventeenth century. During that period, everyone believed in Newton's corpuscular theory. According to this theory, light consists of a stream of minute invisible particles, called corpuscles, moving at a great speed. These corpuscles are emitted in straight lines from a luminous source and their mechanical impact on the retina stimulates the sensation of vision. Different colors were ascribed to different sized corpuscles. On the basis of this theory, the phenomena of reflection, refraction, the rectilinear propagation of light, and the fact that light could propagate through a vacuum were explained satisfactorily. But the corpuscular theory could not explain the phenomenon of interference and diffraction of light. These phenomena could be explained only when the wave theory of light was considered.
The wave theory of light was first proposed by Huygens in 1678. On the basis of his wave theory, Huygens explained satisfactorily the phenomena of reflection, refraction and total internal reflection. According to his theory a luminous body is a source of disturbance in a hypothetical medium called ether. This medium pervades all space. The disturbance from the source is propagated in the form of waves through space and the energy is distributed equally in all directions. When these waves carrying energy are incident on the eye, the optic nerves...
