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) Given : Power of the bulb = 100 W

Since, 5% of the power of a 100 W light bulb is converted to visible radiation, therefore the power of visible radiation can be calculated as follows:



P’ = 5W


The average intensity of visible radiation at a distance of 1m from the bulb can be calculated by using the following formula:



⇒


⇒


⇒ I = 0.398 W/m2


(b) The average intensity of visible radiation at a distance of 10 m from the bulb can be calculated by using the following formula:


I = P’/4π d2


I =


I = 5/1256


I = 0.00398 W/m2


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