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A point source of light S is placed on the axis of a lens of focal length 20cm as shown. A screen is placed normal to the axis of lens at a distance x from it. Treat all rays as paraxial.

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Intensity at centre of screen for x=90cm and x=110cm is same.

As x is increased from zero intensity first increases then decreases.

As x is increased from zero intensity continuously decreases.

Radius of bright circle obtained on screen is equal to 1cm for x=200cm

answer is B, C, D.

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Detailed Solution

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$\Rightarrow \frac{1}{v} - \frac{1}{(- 25)} = \frac{1}{20}$

$\Rightarrow \frac{1}{v} = \frac{1}{20} - \frac{1}{25} = \frac{1}{100}$

$\Rightarrow v = 100 c m$

From O to I intensity incrases and then decreases
At x=90cm and 110cm intensity is same.
(d) Radius at x=200cm is equal to radius of lens.

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