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Question

A point source emitting light uniformly in directions is placed 60 cm above-top. The illuminance at a point on the table-top , directly below the source, is 15 lux .Find the illuminance at a point on the table-top 80 cm away from the first point.


Solution

Given that :

EA=15 lux=I0(60)2

I0=15×(0.6)2

=5.4 candela

So,    EB=10 cos θ/OB2

=5.4(35)12=3.24 lux

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