An object is at a distance of 20 m from a convex lens of focal length… | JEE Main 2019 (11 Jan, Shift 1)

Q93JEE Main 2019 · 11 JAN · SHIFT 1Medium

Spherical Surfaces & Thin Lenses

An object is at a distance of 20 m from a convex lens of focal length 0.3 m. The lens forms an image of the object. If the object moves away from the lens at a speed of 5 m/s, the speed and direction of the image will be:

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Answer: (a)

$\frac{1}{v} = \frac{1}{0.3} - \frac{1}{20} = 3.283 \Rightarrow v \approx 0.3046$ m
Image speed: $|v_i| = \frac{v^2}{u^2}|v_o| = \left(\frac{0.3046}{20}\right)^2 \times 5$
$|v_i| \approx 1.16 \times 10^{-3}$ m/s
As the object moves away the image moves towards the focus, i.e. towards the lens. So the answer is (a).

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