A bulb is located on a wall. Its image of the same size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance d ahead of the second wall, then the required focal length will be
- only d4
- only d2
- more than d4 but less than d2
- less than d4
Answer
(2) only d2
Solution
Image is real and of the same size ⇒ |m| = |v||u| = 1
⇒ |u| = |v|
∵ The image is formed on the second wall, which is at distance d from the lens:
v = +d, u = -d
Lens formula:
1f = 1v - 1u
= 1d + 1d
= 2d
∴ f = d2
Answer: (2)