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Question
Define the term principal foci as applied to a concave lens. Illustrate your answer with the aid of proper diagrams. Show the focal length of the lens in the diagram.
Solution
A concave lens has two principal foci:
(i) first focus and
(ii) second focus.
(i) First focus: First focus of a concave lens is point F1 on the principal axis of the lens such that the rays of light appearing to meet at it, after refraction from the lens becomes parallel to the principal axis. This is shown in the figure. The distance of first point F1 from the optical centre O of the lens (i.e., OF1) is the first focal length f1.
(ii) Second focus: Second focus of a concave lens is the point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis of the lens after refraction from the lens appears to be coming from it. This is shown in the figure. The distance of the second focal point F2 from the optical centre O of the lens (i.e., OF2) is called the second focal length f2.
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