1.

e of an object formed by a lens is of magnification -1. If the distance between the objecte is 60 cm, what is the focal length of the lens? If the object is moved 20 cm towards theians where would the image be formed? State reason and also drawa ray diagram in support of your[CBSE (AL) 2016

Answer»

magnification, m = -1 = v/u

[ here magnification is negative. it means lens must be convex lens. ]

or, v = -u .........(1)

a/c to question,

v + (-u) = 60cm ..........(2)

from equations (1) and (2),

-u -u = 60cm

or, -2u = 60cm

or, u = -30cm

v = 30cm

use lens maker formula, 1/v - 1/u = 1/f

or, 1/30 - 1/-30 = 1/f

or, 1/60 = 1/f => f = 60cm

again, if the object is moved 20cm towards the lens .

u = -(30 - 20) = -10cm

f = +60cm

use lens maker formula, 1/v - 1/u = 1/f

or, 1/v - 1/(-10) = 1/60

or, 1/v = 1/-10 + 1/60 = (-6 + 1)/60

or, v = -12cm

hence, image is formed left side of lens


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