Chapter 9

NCERT
Class 10
Science
Solutions
2. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Question:

A convex lens forms a real and inverted image of a needle at a distance of 50cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Answer:

m = –1, v = +50, u = ?

m = v/u

-1 = 50/u

u = 50/(-1)

u = -50

Using lens formula,

1/f = 1/v - 1/u

    = 1/50 - 1/(-50)

    = 2/50

f = 25cm

Power of lens, P = 1/f

                           = 100/25 m-1

                            = +4 D

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