A thin lens focal length `f` and its aperture has diameter `d`. It forms an image of intensity `I`. Now the central part of the aperture up to diameter`(d)/(2)` is blocked by an opaque paper. The focal length and image intensity will change to

Options

A. `f/2` and `1/2`
B. f and `1/4`
C. `(3f)/4` and `1/2`
D. f and `(3I)/4`

Answer

4

Explanation

Centre part of the aperture up to diameter `d/2` is blocked i.e., `1/4` th area is blocked `(A=(pid^(2))/4)`
Hence remaining area `A. = 3/4 A`. Also, we know that intensity `propto` Area `impliesI^(.)/I=A^(.)/A=3/4 impliesI^(.)=3/4 I`
Focal length doesn’t depend upon aperture.