A point object is placed at a distance of 25 cm from a convex lens of focal length 20 cm. If a glass slab of thickness $t$ and refractive index 1.5 is inserted between the lens and the object, the image is formed at infinity. The thickness $t$ is

Question

Dr. Abubacker · Accepted Answer

(a) In the situation given, the image will be formed at infinity, if the object is at focus of the lens $ie,$ at 20 cm from the lens. Hence, shift in position of object
$x = 25 - 20 = \left( 1 - \frac{1}{\mu} \right)t$
$5 = \left( 1 - \frac{1}{1.5} \right)t$
$t = 15\ $cm

Q. A point object is placed at a distance of 25 cm from a convex lens of focal length 20 cm. If a glass slab of thickness $t$ and refractive index 1.5 is inserted between the lens and the object, the image is formed at infinity. The thickness $t$ is

Explanation

Reviewed by Dr. Abubacker

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