Question

Calculate the focal length of a convex lens, which produces a virtual image at a distance
of 50 cm of an object, placed 20 cm in front of it.
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Answer 1

Answer:

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Explanation:

Given,

Image distance , v = - 50 cm

( Since , the image is virtual in case of a convex lens )

Object Distance , u = -20cm

Focal length , f = ?

By applying the formula for lenses we have,

1/f = 1/v - 1/u

= 1 / (-50) - 1/ (-20)

= 1 / (-50 ) + 1/ 20

= -2 + 5 / 100 ( By L.C.M )

= 3 / 100

Therefore, f = 100/3 cm or 33.34 cm (approx.)

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Answer 2

Answer:

F = 33.3

Explanation:

V = 50cm (virtual image)

u= -20 cm

1/v-1/u = 1/f

1/-50 - 1/-20 = 1/f

(-2+5)/100 = 1/f

3/100 = 1/f

F = 33.3