First slide

Ray optics

Question

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

Easy

A

10 cm
B

5 cm
C

20 cm
D

15 cm

Solution

For the image to form at infinity the object must be at focus of the lens and hence the shift is 25 - 20=5 cm. We know $5 = t (1 - \frac{2}{3}) \Rightarrow t = 15 c m$

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