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A point source of light is kept at a di...

A point source of light is kept at a distanceof`15cm`from a converging lens,on its optical axis.The focal length of the lens is`10cm`and its diameter is`2cm` ,A secreen is placed on the other side of the lens ,perpendicualr to the axis fo lens,at a distance`20cm`from it.Then find the area of the illuminated part of the screen?

Text Solution

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The correct Answer is:
`(pi//4)cm^(2)`

`(1)/(v)-(1)/(u)=(1)/(f)rArr(1)/(v)-(1)/(-15)=(1)/(10)rArr (1)/(v)=(1)/(10)-(1)/(15)`
`(1)/(v)=(3-2)/(30) rArr v=30cm`
for r,`(1.5)/(30)=(r )/(10)rArr r=(1)/(2)cm rArr A=pir^(2) rArr A=pi((1)/(2))^(2)`
`A=(pi)/(4)cm^(2)`
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