Question

for an object placed at a distance of 30cm infront of a convex lens, the image is at distance 30cm behind the lens the focal length of the lens is
(a)15cm (30cm (c)60cm (d)45cm​

Answer 1

Object distance, u = - 30 cm (from sign convention)

Image distance, v = ?

Focal length of the lens f = 20 cm

From lens formula

1f=1v−1u

120=1v−1−30

120=1v+130

1v=120−130

1v=30−2030×20=10600

v=60010=60 cm .

Magnification, m = height of the imageheight of the object =vu

M=60−30=−2.

Negative sign indicates that the image is inverted . Since, m is greater than one, the image is magnified.

Since v is positive, the image is real.

The image is formed 60 cm on the other side of the lens and it is real, inverted and magnified.

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