Q. 145.0( 1 Vote )

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 = √(2.42+1.82) = √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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