For a plane wave the intensity (power per unit area) has a magnitude:
| I |
= |
| EB
|  |
| mo
|
|
B = E/c, so the magnitude of the instantaneous intensity is:
| I |
= |
| E2
| |
| c mo
|
|
= |
| c B2
| |
| mo
|
|
The average intensity is:
| Iav |
= |
| ErmsBrms
| |
| mo
|
|
= |
| EmaxBmax
| |
| 2mo
|
|
= |
| Emax2
| |
| 2c mo
|
|
= |
| c Bmax2
| |
| 2 mo
|
|
For example, a 3 mW laser with a beam with a radius of 1 mm has an average intensity of 955 W/m2. This comes from dividing the power in watts by the beam area in m2. Compare this to sunlight, with an average intensity of a little over 1000 W/m2, and you see why it's dangerous to shine a laser beam in your eye.
The peak electric field in the laser beam is Emax = 0.05 V/m.
The peak magnetic field is Bmax = 1.6 x 10-10 T.
Energy density
The energy density, in units of J/m3, is:
| u |
= |
uE + uB |
= |
| eoE2
| |
| 2
|
|
+ |
| B2
| |
| 2 mo |
|
= |
eoE2 |
= |
| B2
| |
| mo
|
|
Exactly half the energy in an EM wave is in the electric field, and half in the magnetic field.
To get from energy density (J/m3) to intensity (J/(s m2)) multiply by c (m/s).
I = cu