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Question 8.12: About 5% of the power of a 100 W light bulb is converted to visible radiation. What is the average intensity of visible radiation (a) at a distance of 1 m from the bulb? (b) at a distance of 10 m? Assume that the radiation is emitted isotropically and neglect reflection.Class 12 - Physics - Electromagnetic Waves Electromagnetic Waves Page-287

Answer»

The bulb as a point source, radiate light in all DIRECTION uniformly and it is given that only 5% of power is converted to visible radiations.

(a) let a SPHERE of RADIUS , r = 1 m
surface area , A = 4πr² = 4π(1)² = 4π m²
now, intensity = power/area
given, Power = 5 % of 100W = 5W
and area = 4π m²
so, intensity = 5/4π = 5/(12.42) = 0.4 W/m²

(b) let a sphere of radius, R = 10m
surface area , A' = 4πR² = 4π(10)² = 400π m²
intensity = power/area
= 5% of 100W/400π
= 5/400π = 0.004 W/m²


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