Question

In order to obtain a real image image thrice the size of the object with a concave mirror of focal length 20cms, the object should be placed (1 Point) a)At 60cms from the pole
b)At 20 cms from the pole
c)At midpoint of F and C
(d) Both b and c​

Answer 1

Answer:

It is given that,

Focal length

f=−20cm

hi=3ho

The image is three times the size of the object. So there are two cases.

Case 1. For real image

m=−uv=−3

v=3u

Using mirror’s formula

f1=v1+u1

−201=3u1+u1

u= −80/3 cm

Case 2. For virtual image

m=u−v=+3

v=−3u

Again using mirror’s formula

−201=−3u1+u1

u= −40/3 cm

The possible values of u are −80/3 cm and −40/3cm.

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