A 20W light bulb emits a continuous stream of light in all directions. At a part

Question

A 20W light bulb emits a continuous stream of light in all directions. At a particular distance away from bulb, the squareroot mean squared electric field has a strength of 12.5 mu N/C. What is the squareroot mean squared magnetic field strength at this point? What is the average total energy density at this point? What is the intensity of the light at this point? How far away from the light bulb is our point of interest? How far back in time would a person standing at the point of interest see the light bulb?

Explanation / Answer

a) Brms = Erms/c

= 12.5*10^-6/(3*10^8)

= 4.17*10^-14 T

b) Average total energy density = epsilon*Erms^2 + Brms^2/mue

= 8.854*10^-12*(12.5*10^-6)^2 + (4.17*10^-14)^2/(4*pi*10^-7)

= 2.77*10^-21 J/m^3

c)

Intensity of light wave, I = Erms^2/(c*mue)

= (12.5*10^-6)^2/(3*10^8*4*pi*10^-7)

= 4.14*10^-13 W/m^2

d) we know, Intensity = Power/Area

I = P/(4*pi*d^2)

d^2 = P/(4*pi*I)

d = sqrt(P/(4*pi*I))

= sqrt(20/(4*pi*4.14*10^-13))

= 1.96*10^6 m

e) t = d/c

= 1.96*10^6/(3*10^8)

= 6.53*10^-3 s

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