Question

A 60-W light bulb radiates electromagnetic waves uniformly in all directions. At a distance of 1.0 m from the bulb, the light intensity is I0, the average energy density of the waves is u0, and the rms electric and magnetic field values are E0 and B0, respectively.
1. At 2.0 m from the bulb, what is the light intensity?

2. At 2.0 m from the bulb, what is the rms magnetic field value?

3. At 2.0 m from the bulb, what is the average energy density of the waves?

Answer 1

1. If we increase the distance to twice it's original value, the light intensity is reduced by one-fourth, the light intensity would be:
I0/4

2. rms magnetic field is inversely proportional to distance, so the new rms magnetic field would be:
B0/2

3. average energy density is inversely proportional to the square of the distance, so the new average energy density is:
E0/4