Distance of the source from the table-top (r) = 60 cm or 0.6 m
Let be the intensity of illumination.
Illuminance directly below the source is given by,
Let be the illuminance at a point 80 cm away from the initial point.
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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