NCERT Class XII Chapter
Electromagnetic Waves
Questions With Solutions
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Question : 12
Total: 15
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 1m from the bulb?
(b) at a distance of 10 m?
Assume that the radiation is emitted isotropically and neglect reflection
(a) at a distance of 1m from the bulb?
(b) at a distance of 10 m?
Assume that the radiation is emitted isotropically and neglect reflection
Solution:
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 us assume a sphere of radius 1 m.
Surface area A = 4πr2 = 4π (1)2 = 4 π m2
Intensity on the sphere
I =
W m − 2
(b) Let us assume a sphere of 10 m radius.
Surface area A =4 π r 2 ( 10 ) 2 m 2
Intensity on the sphere
I =
m − 2
(a) Let us assume a sphere of radius 1 m.
Surface area A = 4πr2 = 4π (1)2 = 4 π m2
Intensity on the sphere
I =
(b) Let us assume a sphere of 10 m radius.
Surface area A =
Intensity on the sphere
I =
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