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.

Answer 1

(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