Q10 of 38 Page 454

A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as

Let d be the distance between the parallel straight lines and the angle between them is θ.



Intensity is given by,



Using above figure, we get







So, correct option is C.

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