Question

an obejct is placed 60 cm in front of concave lense and its focal lenght 30 cm find it image distance......​

Answer 1

Given,

u=30 cm

f=20 cm

v=?

Here,

1/u + 1/v = 1/f

1/30 + 1/v = 1/20

1/v = 1/20 - 1/30

1/v = 1/60 (use calculator )

v= 60 cm

( u, v, f are always in ‘+’ in concave.

But ‘-’ in convex)

Now,

(Since image is formed on the same side of the object , the image is real)

v-u = 60 - 30 = 30 cm

So the distance between object and image is 30 cm.

Answer 2

u = - 60 cm ( sign convention for concave lens)

f = -30 cm ( same reason)

1/f = 1/v - 1/u

1/f + 1/u = 1/v

1/v = 1/-30 + (1/-60)

1/v = 1/-30 -1/60

1/v = -2 -1/60. (L.C.M)

1/v = -3 / 60

v = 60/-3

v = -20 cm

Image distance = -20cm