Rectilinear Propagation, Reversibility and Independence of Light Rays

This article discusses the following three basic laws or principles obeyed by light rays (in ray optics or ray approximation of light waves):
  1. Law of Rectilinear Propagation of Light Rays
  2. Principle of Reversibility of Light Rays
  3. Law of Independence of Light Rays

Law of Rectilinear Propagation of Light Rays

It states that a light ray always propagates along straight line in transparent homogeneous medium or in vacuum. We call this as law of rectilinear (straight line) propagation of light. The light ray may deviate from its path only when there is some change in the homogeneity of the medium or change in refractive index of the medium (or simply, change in medium).

Thus, a light ray which starts from a point A and passes through another point B in the same medium actually crosses all the points lying on the straight line AB.

Principle of Reversibility of Light Rays

According to this principle, the path of light ray is reversible i.e. if the direction in which light ray is travelling is reversed, it will retrace its path. In other words, if by some suitable device, say a mirror, the direction of ray is reversed the ray will accurately retrace its path irrespective of the number of reflections or refractions it might have suffered before its reversal.

Observe the diagram show below. A ray AO is incident on a plane reflecting surface and OB is the corresponding reflected ray. By the principle of reversibility of light, if BO would incident ray then OA would be the corresponding reflected ray.

Law of Independence of Light Rays

This states that light rays do not disturb each other upon intersection. Path of one light ray is independent of the presence of other light ray(s).


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