Question

9. Find focal length of a lens which produces a virtual
image at a distance of 50 cm of an object placed at 20 cm
from it.​

Answer 1

Explanation:

Hey mate..

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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.)

Answer 2

v=50cm u=-20cm

1/f=1/v-1/u

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

=1/50+1/20

=2+5/100

f=100/7

f=14.29cm