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A screen is placed at a distance of 1m f...

A screen is placed at a distance of 1m from a point of source of light . What will be the effect on the intensity (I) of light observed on the screen , if the screen is kept at 150 cm from the same point source ?

A

1 increases by 50%

B

1 decreases by 40 %

C

1 decreases by 55.55%

D

1 increases by 55.55%

Text Solution

Verified by Experts

The correct Answer is:
C
For a point source , `I prop (1)/(d^(2))`
In this case , the distance is increased to 150 cm
=1.5 m `= (3)/(2)` m
`therefore I_(2) prop (1)/((3//2)^(2)) therefore (I_(2))/(I_(1)) = (1^(2))/(9//4) = (4)/(9)`
`therefore I_(2) = (4)/(9) I_(1) therefore I_(1) - I_(2) = I_(1) - (4)/(9) I_(1) = (5)/(9) I_(1)`
`therefore` The intensity is decreased by `(5)/(9) xx 100 = (500)/(9) = 55.55%`
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