A small lamp is hung at a height of 8 feet above the
centre of a round table of diameter 16 feet. The ratio
of the intensities of illumination at the centre and at a
point on the circumference of the table will be:
(A) 1:1
(B) 2.2 :
(C) 1:3
(D) 3:2
Answers
Answer:
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Answer:
Illuminance=illuminating power∗Cosθ/d
2
The only change when lamp moves from 4 m to 3 m is its d
Illuminance at 4 m = Power / 16
Illuminance at 3 m = Power / 9
Increase in Illuminance =
9
power
−
16
power
=
144
7Power
So % increase =
144
7Power
∗
Power
16∗100
= 77.77 %