One hard and stormy night you find yourself lost in the forest when you come upon a small hut. Entering it you see a crooked old woman in the corner hunched over a crystal ball. You are about to make a hasty exit when you hear the howl of wolves outside. Taking another look at the gyspy you decide to take your chances with the wolves, but the door is jammed shut. Resigned to a bad situation you approach her slowly, wondering just what is the focal length of that nifty crystal ball.
The image of old lady is

Options

A. 10cm
B. 5cm
C. 15cm
D. None of these

Answer

B

Explanation

at this point image will formed at infinity
For refraction at second surface `(1)/(oo) - (1.5)/(v_(1)) = (1-1.15)/(-10) implies v_(1) = -30"cm"`
For refraction at first surface `(1.5)/(-10) - (1)/(x) = (1.5-1)/(+10)implies x=5"cm"`