1.

A lensof negligible thickness of focal length (f) has as aperture (d). If forms an image of intensity I. Now, the central partof aperture upto diameter d/2 is blocked by opaque paper. The focal length and image intensity will change to :

Answer»

`(F)/(2),(I)/(2)`
`(3f)/(4),(I)/(2)`
`f,(I)/(4)`
`f,(3I)/(4)`

SOLUTION :(d) The focal length of lens does not change if a part of it is blocked, if central part of aperture upto `(d)/(2)` is blocked.
`therefore` EXPOSED area of aperture reduces by one FOURTH the initial areal.
`therefore (pi(d//2)^(2))/(pid^(2))=(1)/(4)`
Intensity of image reduced factor of 4.
`therefore "" I. = I - (1)/(4) I = (3I)/(4).`


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