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Question
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.
Solution
(i) A ray of light incident at the optical centre O of the lens passes undeviated through the lens.
Convex lens
Concave lens
(ii) A ray of light incident parallel to the principal axis of the lens, after refraction passes through the second focus F2 (in a convex lens) or appears to come from the second focus F2 (in a concave lens).
Convex lens
Concave lens
(iii) A ray of light passing through the first focus F1 (in a convex lens) or directed towards the first focus F1 (in a concave lens), emerges parallel to the principal axis after refraction.
Convex lens
Concave lens
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