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A point sources of light is 60cm from a ...

A point sources of light is `60cm` from a screen and is kept at the focus of a concave mirror which reflect light on the screen .the focal length of the mirror is `20cm`The ratio of average intensities of the illiumination on the screen when the mirror is present and when the mirrror is removed is:

A

`36:1`

B

`37:1`

C

`49:1`

D

`10:1`

Text Solution

Verified by Experts

The correct Answer is:
D

`I_(1) = "intensity due to source directly"`
`I_(2)= "Intensity due to reflected light" rArr I=(P)/(4pir^(2))`
`(I_(1))/(I_(2))=((r_(2))/(r_(1)))^(2)=(20/60)^(2) rArr(I_(1))/(I_(2))=(1)/(9)`
`(I_(1)+I_(2))/(I_(1))=(10)/(1)`
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