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Question

At what distance from a concave mirror of focal length 10 cm should an object be placed so that:

(a) its real image is formed 20 cm from the mirror?
(b) its virtual image is formed 20 cm from the mirror?

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Solution

(a)
Given,It is a concave mirror. f=-10 cmv=-20 cm Since the image is real and forms in front of the mirror, the equation will be1f=1v+1u 1-10=1-20+1u 1u=1-10+120 1u=-20 +10200 1u=-10200 1u=-120 u=- 20 cm Therefore, the object should be placed at a distance of 20 cm from the mirror to form a real image.

(b)
Given,f=-10 cm v=+20 cm Since the image is virtual and forms behind the mirror, the equation will be1f=1v+1u 1-10=120+1u -110-120=1u 1u=-20-102001u=-302001u=-320u=-203
Therefore, the object must be placed at a distance of 203cm from the mirror to form the virtual image.

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