What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens ? Draw diagrams to support your answer.

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Solution

(i) A ray of light incident at the optical centre O of the lens passes undeviated through the lens.
(ii) A ray of light incident parallel to the principal axis of the lens, after refraction, passes through the second focus (in a convex lens) or appears to come from the second focus (in a concave lens).
(iii) A ray of light passing through the first focus (in a convex lens) or directed towards the first focus (in a concave lens), emerges parallel to the principal axis after refraction.

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