A ray of light is directed towards the principal focus F₁ on the left side of a convex lens but strikes the l…

Question

A ray of light is directed towards the principal focus F₁ on the left side of a convex lens but strikes the lens before actually reaching F₁. Draw the path of this ray after refraction through the lens and explain, using the principle of reversibility of light, why it travels in that direction.

Accepted Answer

## Model Answer After refraction, the ray emerges **parallel to the principal axis**. By the principle of reversibility, a ray parallel to the principal axis passes through F₂ after refraction; reversing this, a ray directed towards F₁ must emerge parallel to the principal axis. Source: Chapter 9, Section 9.3.5 --- ## Explanation The key ray rule here is rule (ii): a ray through F₁ emerges parallel to the principal axis. Reversibility means if you reverse the emergent ray (parallel), it retraces to pass through F₁ — so any ray *directed toward* F₁ (even if it hits the lens first) must emerge parallel. Examiners expect you to name the direction ("parallel to principal axis") and explicitly mention the reversibility principle.

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