Question

Two convex lenses of focal lengths 20 cm and 40 cm and a concave lens of focal length 10 cm are placed as shown below (refer to image). An object is placed at a distance of 20 cm. The value of x and y, for which final rays comes out parallel to the principal axis, are?

Answer 1

Answer:

x can be anything, but y is always 30cm

Explanation:

A concave lens can produce parallel rays when the incident rays are directed towards the second focus, i.e, if we have the image formed by the second lens at the focus (F2) of the third, then the refracted rays will be parallel. Therefore we need to find out the image distance of L2.

By using: 1/f = 1/v- 1/u

u = infinity, 1/u = 0

1/40 = 1/ (y+10)

Therefore, y = 30

hence for all real values of x and y=30, the emeging rays will be parallel to each other.

Answer 2

Explanation:

Hence values of: x=30 cm and y=30cm.