Optics Question 448
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
Options:
A) 10 cm
B) 5 cm
C) 20 cm
D) 15 cm
Show Answer
Answer:
Correct Answer: D
Solution:
[d] Image will be formed at infinity if object is placed at focus of the lens, i.e., at 20 cm from the lens. Hence, Shift = 25 - 20 $ =( 1-\frac{1}{\mu } )\mu $ Or $ 5=( 1-\frac{1}{1.5} )t $ or $ t=\frac{5\times 1.5}{0.5}=15cm $
