Question

A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16cm?

Answer 1

Given :

a) Here the point P acts as virtual object.

object distance=u=12cms

Focal length of object from convex lens=f=+20m

By using the formula :

1/v-1/u=1/f

1/v=1/u+1/f

=1/20+1/2

=3+5/60

=8/60

v=7.5cm

Thus the beam converges on the right side of lens at a distance of 7.5cm

b) Distance of object from lens =u=12cm

focal length of concave lens = -16cm

using lens formula :

1/v-1/u=1/f

1/v=1u+1/f

=1/12-1/16

=4-3/48

Thus the beam converges on the right side of lens at a distance of 48cm

Answer 2

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Refer to the attachment!!