In the following, assume that lightbulbs radiate uniformly in all directions and

Question

In the following, assume that lightbulbs radiate uniformly in all directions and that 5.0% of their power is converted to light.

A) Find the average intensity of light at a point 2.0m from a 120-W red lightbulb ( = 710 nm).

B) Is the average intensity 2.0 m from a 120-W blue lightbulb ( = 480 nm) greater than, less than, or the same as the intensity found in part A? Please EXPLAIN.

C) Calcuate the average intensity for part B.

Complete answers with step by step detail would be greatly appreciated! Thanks!

Explanation / Answer

A) P=120 W
so power of light=Pl=5% of 120 W=5x120/100 W=6 W
so average intensity=I=Pl/A where A=surface are of the sphere at radius R=2m

Thus I=Pl/(4R2)=6/(4x22)=0.1194 W/m2

B)Intensity as calculated above is independent of the frequency/wavelength of the light emitted. It only depends on the strength of the electric field of the light and is related to the amplitude of the electric field as I=E2 where E=amplitude of the electric field of light.

C)

P=120 W
so power of light=Pl=5% of 120 W=5x120/100 W=6 W
so average intensity=I=Pl/A where A=surface are of the sphere at radius R=2m

Thus I=Pl/(4R2)=6/(4x22)=0.1194 W/m2

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