Playing with an old lens one morning, Ravi discovers that if he holds the lens 10 cm
away from a wall opposite to a window he can see a sharp but upside down image of
outside world on the wall. That evening he covers a lighted lamp with a piece of
opaque paper on which he has pierced a small hole 1 mm in diameter. By placing the
lens between the illuminated paper and the wall, he manages to produce a sharp image
of diameter 5 mm on the wall.
1. What is the nature of the lens?
2. What is the focal length of the lens?
3. Calculate the power of the lens.
4. In the evening experiment how far from the opaque paper did he place the lens?
5. How far apart were the illuminated paper and the wall?
Dear Student,
In the first case, we can consider that light came from infinity and focussed on the wall. So the focal length of the lens is 10 cm.
And Since an inverted image is formed (real), it is a convex lens.
Power of lens = 100/10 = 10 D
In the first case, we can consider that light came from infinity and focussed on the wall. So the focal length of the lens is 10 cm.
And Since an inverted image is formed (real), it is a convex lens.
Power of lens = 100/10 = 10 D