Question

The light from a regular lightbulb is visible light. It is estimated only 10% of the energy will be transformed into the visible light and the rest is heat. Heat is coming from the infrared light. What is the approximate magnitude of the infrared magnetic field (in uT)?

Answer 1

Solution:

Let us consider a 100 W light bulb. Only 10% of the energy emitted is visible light i.e, 90% of the energy is infrared.

1. The power of the infrared radiation is given by;

     

2. We have to determine the intensity of infrared radiations at a distance R from the light bulb. For that consider a virtual sphere that has a radius R.

The intensity at the surface of the virtual sphere is given by;

  

where, is the power.

is the surface area of the sphere.

But, .

Let us take normal room with .

Thus,

  

  

3. Calculate the average energy density of the infrared radiation.

The average energy density is given by;

  

where is the intensity and is the speed of light in air.

Thus, we have,

  

  

3. Calculation of approximate magnitude of infrared magnetic field.

The maximum value of the magnetic field is given by;

....................(1)

where is the permeability of free space & is the average energy density.

Substituting and in (1).

we get,