Size of image of an object formed by a mirror having focal length 20 cm is observed to be reduce to 1/3 of its size. At what distance the object has to be placed from the mirror ? What is the nature of the image and the mirror ?

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Solution

the image size has reduced to 1/3 rd of the original object.

Therefore m= v/u = 1/3

using the relation $\frac{1}{u} + \frac{1}{v} = \frac{1}{f}$ for a concave mirror

we get u=4f= 80 cm and v= 80/3 cm which satisfy the reduction in magnification

as u, v and f is having same sign, so the mirror is a concave mirror and image is real but inverted.

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(1) An object of size 7 cm is placed at 25 cm in front of concave mirror of focal length 15 cm. At what distance from the mirror should a screen be placed, so that we can get a sharp and clear image. Find nature and size of the image.

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