A thin lens has focal length f and aperture radius r. It forms an image of intensity I. Now the central part of the aperture upto r 2 is blacked by an opaque material. The focal length and intensity of image will be Options: A . &nbspf2&I2 B . &nbspf&I4 C . &nbsp3f4&I2 D . &nbspf&3I4

Answer: Option B : B There will be no changein focal length. f 1 = f But Intensity I ∝ A But Intensity I ∝ A I 1 I = r 2 1 r 2 = 1 4 ∵ r 1 = r 2 I 1 = I 4

A thin lens has focal length f and aperture radius r. It forms an image of

Question

A thin lens has focal length f and aperture radius r. It forms an image of intensity I. Now the central part of the aperture upto

\frac{r}{2}

is blacked by an opaque material. The focal length and intensity of image will be

Options:

A . f2&I2

B . f&I4

C . 3f4&I2

D . f&3I4

Answer: Option B
:
B
There will be no changein focal length.

f^{1} = f

But Intensity

I \propto A

But Intensity

I \propto A

\frac{I^{1}}{I} = \frac{r_{1}^{2}}{r^{2}} = \frac{1}{4} ∵ r_{1} = \frac{r}{2} I^{1} = \frac{I}{4}

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