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  4. 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

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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

Ray Optics and Optical Instruments

Solution:

In the situation given, the image will be formed at infinity, if the object is at focus of the lens
i.e., 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 $
$t=15 \,cm$