Optics Question 654

Question: A candle placed 25 cm from a lens, forms an image on a screen placed 75 cm on the other end of the lens. The focal length and type of the lens should be

[KCET 2000]

Options:

A) + 18.75 cm and convex lens

B) - 18.75 cm and concave lens

C) + 20.25 cm and convex lens

D) - 20.25 cm and concave lens

Show Answer

Answer:

Correct Answer: A

Solution:

$ u=-25cm,v=+75cm $

$ \Rightarrow $ $ \frac{1}{f}=\frac{1}{+75}-\frac{1}{-25}\Rightarrow f=+18.75 $ cm ; convex lens.



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