Geometrical OpticsMedium
Question
A lens have focal length f and and aperture of diameter d forms an image of intensity I. Aperture of diameter 
in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively
in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively Options
A.f and 
B.
and 
and C.f and 
D.
and 
and Solution
Focal length of lens remains same
Intensity of image formed by lens is proportional to area exposed to incident light from objrct
i.e., Intensity ∝ area.
or
After blocking, exposed area
∴
or I2 =
(∵ I1 = I)
Hence, focal length of lens = f, intensity of the image =
Intensity of image formed by lens is proportional to area exposed to incident light from objrct
i.e., Intensity ∝ area.
or
After blocking, exposed area
∴
or I2 =
(∵ I1 = I)Hence, focal length of lens = f, intensity of the image =
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