Question

A beam of light converges at point P now a convex lens is placed in the path of the convergent beam at 15 cm from a point P at what distance does a beam converges if the convex lens has a focal length 10 cm

Answer 1

At a distance of 6cm the beam converges if the convex lens has a focal length 10 cm

Given, the focal length is 10cm.

So, f = 10cm

As  

We know,

1/f = 1/v -1/u

Distance of convex lens from point P (u) = 15cm

1/v -1/15= 1/10

⇒  1/v =1/10 +1/15 = 5/30  = 1/6

⇒  v = 6 cm

So, the image is at 6 cm to the right of the lens where the beam converges.