A thin lens focal length `f_(1)` 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),(I)/(2)`
B. `f,(I)/(4)`
C. `(3f)/(4),(I)/(2)`
D. `f,(3I)/(4)`

Answer

D

Explanation

The focal length remains same
`Iproppi((d)/(2))^2`
`I' proppi[((d)/(2))^2-((d)/(4))^2]`
`(I')/(I)=1-(1)/(4)=(3)/(4)impliesI'=(3)/(4)I`