Question

Find the magnification of the lens if the focal length of the lens is 10 cm and the size of the image is -30 cm.

(a) 2

(b) 3

(c) 4

(d) 5

This question was posed to me in final exam.

I would like to ask this question from Ray Optics in section Ray Optics and Optical Instruments of Physics – Class 12

Answer 1

The correct option is (a) 2

For explanation: Given: f = +10 cm; v = -30 cm

Required equations ➔

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

m=\(\frac {size \, of \, the \, image}{size \, of \, the \, object}=\frac {v}{u}\)

\(\frac {1}{u}=\frac {1}{v}-\frac {1}{f}=\frac {1}{-30}-\frac {1}{10} = \frac {-(10+30)}{300} = \frac {-40}{300}= \frac {-2}{15}\)

u=\(\frac {-15}{2}\) = -7.5 cm

m=\(\frac {v}{u}= – \frac {30}{-7.5}=\frac {-300}{-75}\)= +4

Therefore, the object should be placed at a distance of 7.5 cm from the lens to get the image at a distance of 30 cm from the lens. It is four times enlarged and is erect.