A convex lens is put 10 cm from a light source and it makes a sharp… | JEE Main 2019 (9 Jan, Shift 1)

Q69JEE Main 2019 · 9 JAN · SHIFT 1Medium

Refraction, Glass Slab & Apparent Depth

A convex lens is put 10 cm from a light source and it makes a sharp image on a screen, kept 10 cm from the lens. Now a glass block (refractive index 1.5) of 1.5 cm thickness is placed in contact with the light source. To get the sharp image again, the screen is shifted by a distance d. Then d is:

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Answer: (d)

Lens: $\dfrac{1}{f} = \dfrac{1}{10} + \dfrac{1}{10} \Rightarrow f = 5$ cm.
Slab shifts the object towards the lens by $t\left(1 - \dfrac{1}{\mu}\right) = 1.5\left(1 - \dfrac{1}{1.5}\right) = 0.5$ cm, so $u = 9.5$ cm.
$\dfrac{1}{v} = \dfrac{1}{5} - \dfrac{1}{9.5} = \dfrac{4.5}{47.5} \Rightarrow v = 10.55$ cm.
Screen moves $0.55$ cm away from the lens, so the answer is (d).

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Refraction, Glass Slab & Apparent Depth question from JEE Main 2019 (9 Jan, Shift 1)

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