Question

We can reasonably model a 120-W incandescent light-bulb as a sphere 6.1 cm in diameter. Typically, only about 6% of the energy goes to visible light; the rest goes largely to nonvisible infrared radiation.

(a) What is the visible-light intensity at the surface of the bulb?
W/m²

(b) What are the amplitudes of the electric and magnetic fields at this surface, for a sinusoidal wave with this intensity?

Emax	=  V/m
Bmax	=  µT

Answer 1

(a) Bulb is radiating uniformly in all directions and that only 6% of the energy goes in to light;

therefore Power (P) = 0.06*120 = 7.2 W

Area (A) = 4*pi*(0.061/2)² m² = 11.688*10^-3 m²

The intensity of light, I = P/A;

I = 616.02 W/m² (visible-light intensity at the surface of the bulb).

(B) Amplitudes of the electric and magnetic fields at this surface

Emax = Square-root (2*I/epsilon*c) ; Where, c = 3*10^8 m/s is the speed of light and epsilon = 8.85*10^-12

Emax = 6812.06 V/m.

Bmax = Emax/c = 6812.06/(3*10^8)

Bmax =2270.68 * 10^-8 T

= 22.70 *10^-6 T.