Q. 11

A point source emitting light uniformly in all directions is placed 60 cm above a table-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.

Answer :

Given,
Distance of the source from the table-top (r) = 60 cm or 0.6 m
Let
be the intensity of illumination.

Capture.PNG
Illuminance directly below the source
is given by,




Let be the illuminance at a point 80 cm away from the initial point.
So,



From the figure, we get


OB = 1 m



Substituting the respective values in the above formula, we get


So, the illuminance at a point on the table-top 80 cm away from the first point is 3.24 lux.


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