Some simple demonstration experiments with laser

Rajpal S Sirohi

(In loving memory of Revered Professor M. S. Sodha)

Passing away of Prof. Sodha had been a greatest loss for me. He was my Ph.D. Supervisor. I learnt Optics from him. All along he had been a great mentor, advisor, supporter and fatherly figure. It is but natural that I prepare a simple note on optics to be included in this memorial issue being brought out by the Asian Journal of Physics.

                Laser beam is a very convenient representation for the ray in geometrical optics. It also serves very well when we use it to study wave related phenomena such as interference, diffraction etc. An unexpanded laser beam travels in a straight line, albite with a little divergence, in a homogeneous and isotropic medium or in free space without magnestic field. All the demonstration experiments except those involving Talbot phenomenon are conducted with an unexpanded laser beam.

                Figure 1(a) shows a plano convex lens on which five parallel rays are incident from the flat side of the lens. Rays closer to the optical axis (central ray) come to focus, which is called the paraxial focal point and the distance from the vertex of the convex surface to the focal point is the back paraxial focal length. The two outer rays intersect the optical axis closer thus showing a strong spherical aberration. Axial spherical aberration is the difference between the paraxial focal length and marginal focal length. Figure 1(b) shows the similar case except that the parallel rays are incident on a convex surface. Notice that the paraxial rays come to focus on the paraxial focal point. Similarly, the outer rays focus at a slightly different point which is closer to the flat surface.  But the magnitude of spherical aberration is much smaller. Following are the take away points;
Doi: 

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