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

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

NTA AbhyasNTA Abhyas 2022Ray Optics and Optical Instruments

Solution:

Solution
For final image at infinity, image shifted by the slab must be at the focus of convex lens.
Therefore, shift $=t\left(1 - \frac{1}{\mu }\right)$
$\Rightarrow 5=t\left(1 - \frac{2}{3}\right)$
$\therefore t=15cm$