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Question 2

What is the shape of the wavefront in each of the following cases:

(a) Light diverging from a point source.

(b) Light emerging out of a convex lens when a point source is placed at its focus.

(c) The portion of the wavefront of light from a distant star intercepted by the Earth.

Answer

(a) Light diverging from a point source.

The geometrical shape of the wavefront in case of light diverging from a point source will be diverging spherical wavefront . It can be shown as,



(b) Light emerging out of a convex lens when a point source is placed at its focus.

When a point source is placed at the focus of a convex lens, the rays emerging from the lens are parallel. Therefore, the wavefront must be plane, as shown in figure,

(c) The portion of the wavefront of light from a distant star intercepted by the Earth.

The source of light (i.e. star) is very far off (i.e. at infinity), the wavefront intercepted by the earth will be a plane wavefront, as shown above.

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