Question

A light beam converges at a point O. In the path of this beam, a concave lens of focal length 15 cm is placed at a distance of 10 cm before point O. The beam now converges at a point O'. Find the magnitude and the direction of shift OO'.

Answer 1

Given: Focal length of convex lens (f) = −15 cm

Distance of the lens from point O (u) = −10 cm

Using the lens formula:

`1/f = 1/v - 1/u`

`1/v = 1/-15 + 1/10`

= `((-2 + 3))/30`

`1/v = 1/30`

v = 30

Shift in convergence OO':

OO' = |v − O|

= |30 − 10|

= 20 cm

Hence, the magnitude of the shift OO' is 20 cm, and the direction is away from the lens (to the right).