A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is
- 10 cm
20 cm
- 30 cm
5 cm
As we learnt in
Thin lens formula -
- wherein
are object and image distance from lens.
u=+10 cm
V=+15 cm
Option 1)
- 10 cm
This is incorrect option
Option 2)
20 cm
This is incorrect option
Option 3)
- 30 cm
This is correct option
Option 4)
5 cm
This is incorrect option
u=+ (15-5) = +10cm ,v=+15cm
1/f=1/v- 1/u
1/15-1/10=-1/30
f= - 30cm