Question

A real image 1/5 th the size of the object is formed at a distance of 18 cm from a lens. From the table select the row containing the correct identification of lens and focal length of the lens. Type of lens. Focal length (a) Concave. - 15cm (b) Concave. -90cm (c) Convex. +30cm dy Convex. +15cm A​

Answer 1

Answer:

since image is real, lens is convex.

v=18

v/u=1/5

18/u=1/5

u=90

As per convention,

u=-90

v=+18

put in the formula

1/v-1/u=1//f

(1/18)-(-1/90)=1/f

6/90=1/f

f=15

f is + ve that also shows that lens is convex

Ans d