Question

The magnification of an image formed by a lens is -1. If the distance of image

from the lens is 25 cm. then find the nature and focal length of lens. ​

Answer 1

Answer:

Focal length of lens = 12.25 cm

Nature of lens : Convex lens.

Explanation:

Given:

Magnification, m = -1.

Image distance, v = 25 cm.

Equation for relation between Magnification, Object distance and Image distance:

Magnification, m = Image distance (v) / Object distance (u).

Substituting given values, we get,

-1 = 25cm /u

That is,

-u = 25cm

So,

Object distance, u = - 25 cm.

Equation for relation between Focal length, Object distance and Image distance:

(1/f) = (1/v) - (1/u)

Substituting the values, we get,

(1/f) = (1/25) - (1/-25) = (1/25) - (-1/25) = [ 1-(-1) ] / 25 = (1+1)/25 = 2/25.

So,

f = 25/2 = 12.25 cm

Therefore,

Focal Length of lens = 12.25 cm

Nature of lens : Convex lens.

(Since magnification is negative, image formed is real and inverted. A real and inverted image can be formed only by a Convex lens. Hence, the answer.)