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 20cm, and (b) a concave lens of focal length 16cm?

Question

Philoid · Accepted Answer

Given data,

The Lens is placed in the path of the convergent beam 12 cm from P.

Here, the object is virtual and the image is real, u = + 12 cm (since, the object on right and virtual)

Reference ray diagram:

a) Given f = + 20 cm for the convex lens.

Using Len’s formula:

⇒

i.e. v = 7.5 cm (image on right and real)

This means that the beam converges as a point 7.5 cms from the lens

b) Here, f = -16 cm

According to the lens formula, we have the relation:

l

⇒

⇒ v = 48 cm

Hence, the image will be located 48 cm from the lens.