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Question

A concave mirror forms an image of 20 cm high object on a screen placed 5.0 m away from the mirror. The height of the image is 50 cm. Find the focal length of the mirror and the distance between the mirror and the object.

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Solution

Given that, h1=20 dm

v = -5 m = -500 cm,

Since vu=h2h1

or 500u=5020

(because the image is inverted)

or u=500×25

= - 200 cm = -2 m

1v+1u=1f

or 15+12=1f

or 1f=710

or f=107=1.44

So, the focal length is 1.44 m.


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