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Question

An electric lamp and a candle produce equal illuminance on a screen when placed 80cm and 20cm from the screen respectively. The lamp is now covered with a thin paper which transmit 49% of the luminous flux. By what distance the lamp should be moved to balance the intensities at the screen again?

A
24cm
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B
12cm
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C
18cm
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D
456cm
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Solution

The correct option is B 24cm
Case(i)
I1I2 = (8020)2
Case(II)
.49I1I2 = (d20)2
d = 20.49(16) = 20(2.8)
= 56cm
Lamp be moved by 80 - 56 = 24 cm

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