Question

A lens has a focal length of 10 cm. Where the object should be placed if the image is to be 40 cm in the positive direction from the lens?

(a) 12 cm

(b) 40 cm

(c) 13 cm

(d) 0.075

I got this question in final exam.

The origin of the question is Ray Optics topic in division Ray Optics and Optical Instruments of Physics – Class 12

Answer 1

Correct choice is (c) 13 cm

The best I can explain: Given: f = + 20 cm; v = + 40 cm

The required equation ➔

\(\frac {1}{f}=\frac {1}{v}-\frac {1}{u}\)

\(\frac {1}{10}=\frac {1}{10}-\frac {1}{u}\)

\(\frac {1}{u}=\frac {1}{10}-\frac {1}{40} = \frac {40-10}{400}\)

\(\frac {1}{u}=\frac {30}{400}=\frac {3}{40}\)

u=\(\frac {40}{3}\)=13.3 cm