Question

siz3 of image or an object formed by a mirror having focal length 20cm is observed to reduced to 1/3 of its size. at what distance object has been placed from the mirror what is the nature of image formed and mirror

Answer 1

A small image can be formed both by convex and concave mirror. So,

In concave :

Given

m = -1\3

f = -20 cm

We know that,

m = -v\u

-1\3 = -v\u

v = u\3 ......................(i)

1\v + 1\u = 1\f

3\u + 1\u = 1\-20

u = -80 cm

substituting eqn (i),

v = u\3 = -80\3

Therefore image is real and inverted.

In convex :

Given

m = 1\3

f = 20 cm

m = -v\u

1\3 = -v\u

v = -u\3 ................(ii)

1\v + 1\u = 1\f

-3\u + 1\u = 1\20

u = -40 cm

Substituting eqn (ii),

v = -(-40)\3 = 40\3 cm

Therefore the image is virtual and erect .

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