An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be

1. 30 cm away from the mirror

2. 30 cm towards the mirror

3. 36 cm away from the mirror

4. 36 cm towards the mirror

Doubtly Asked 30 Oct 2026 NEET UG 2018 Practice test neet Save WhatsApp Report

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Answer: 3. 36 cm away from the mirror

Use the mirror formula \(\dfrac1v+\dfrac1u=\dfrac1f\) with \(f=-15\) cm.

Case 1: \(u=-40\): \(\dfrac1v=-\dfrac1{15}+\dfrac1{40}=-\dfrac{5}{120}\Rightarrow v=-24\) cm.

Case 2: object moves 20 cm closer, \(u=-20\): \(\dfrac1v=-\dfrac1{15}+\dfrac1{20}=-\dfrac1{60}\Rightarrow v=-60\) cm.

The image moves from 24 cm to 60 cm in front of the mirror, so the image shifts 36 cm away from the mirror.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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