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Question
Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4 m with its plane parallel to the table-top. The illuminance at the point Adirectly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8 m from A.
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Solution
Let x be the distance moved.
Let the illuminance of the candle and the lamp at a distance 1 m be IC and ILrespectively. According to question,
`E=I_L/(0.8)^2=I_c/(0.2)^2`
`rArr I_L/I_c=16`
Again after the wrapping the paper around
`49/100I_L/(0.8+x)^2=I_c/(0.2)^2`
Either,
x = -0.24
Or,
x = -1.36 this value is not acceptable because it is more than 0.8
so, x = 0.24m (-ve sign denotes distance is to be decreased)
x = 24cm
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