This question concerna a symmetrical lens shown, along with its two focal points. It is made of plastice with `n=1.2` and has focal length f. Four different regions are shown:
Here,
A. `-oox lt -f` B. `-f lt x lt 0`
C. `0 lt x lt f` D. `f lt x lt oo`

Q. A second lens is now placed adjacent to the first in contact with it. The new lens is concave, with a focal length `f=-3f`. What is the focal length of the two-lens system? Treat the lenses as thin.

Options

A. A
B. B
C. C
D. D

Answer

d

Explanation

From the formula of equivalent focal length of thin lenses in contact, we get
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2))=(1)/(f)-(1)/(3f) or F=(3f)/(2)`