Question

Draw the diagrams to show the behaviour of plane wave fronts as they (a) pass through a thin prism, and (b) pass through a thin convex lens and (c) reflect by a concave mirror.

Answer 1

Hint: Learn the definition of plane front. Learn the different properties of a wave front. A wave front is an imaginary surface that contains all the points of a wave which have the same phase and it moves with the same phase with time if undisrupted. The plane wave front is a plane surface which contains all the points at the same phase.

Complete answer:
We have given her the diagram of a plane wave front when passing through a prism, a lens and a concave mirror. We know that the wave front is an imaginary surface that contains all the points of a wave which have the same phase. And a plane wave front is nothing but a plane that acts as a wave front. Now, to draw the diagram we first need to know how the plane wave front will react when passing through each medium.

(a) Now, at first consider the case of a ray, for a single ray which is nothing but a point wave front when passing through a prism it will only bend by some angle for every height from the prism. So, a plane wave front will also only bend by some angle when passing through the prism. Hence, the diagram is shown as below.

(b) Now, for a convex lens consider a point of the wave front when passing through it, it will always pass through the focus of the lens for any height from the principal axis. For incidence through the principal axis it will go straight through. So, the plane wave front will also change into spherical shape while passing through the lens and will converge all the points at focus. So, the diagram is shown below.

(c) For a concave lens the same thing will happen, the only difference is that the wave front will be reflected back instead of refracted. Hence, the diagram is shown below.

Note: To draw the diagram remember the basic property of a lens or mirror. When parallel waves are incident on a mirror or a lens it always converges to or diverges from the focus. When passing through a convex lens or concave mirror the plane wave front converges at the focus of the lens or the mirror.