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# Figure (22-E1) shows a small diffused plane source S placed over a horizontal table-top at a distance of 24 m with its plane parallel to the table-top. The illuminance at the point A directly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 18 m from A.

Answer :

In triangle ABS,

AS =2.4 m

AB = 1.8 m.

Hence BS = √(2.4^{2}+1.8^{2}) = √9 = 3 m.

Let, ,

Illuminance at A = 25 lux.

Let the intensity along SA =.

Since it is a small extended source, from Lambert's cosine law, the intensity along SB:

I

.

The angle between SB and normal at B is also θ.

Hence the illuminance at B,

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