« on: 15/06/2016 17:34:07 »
I think Einstein also said of general relativity that it was so simple a 7 year old should understand it. Maybe a 7 year old Einstein...
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Light is an electromagnetic (EM) wave whose wavelength lies in the range 390 to 700 nm. Most of the energy in an EM wave is in the electric field component. An EM wave cannot interact with neutrons because their charge is zero and are therefore unable to interact via the electromagnetic interaction. I.e. neutrons cannot couple to an EM field.
The energy in an electromagnetic wave is tied up in the electric and magnetic fields. In general, the energy per unit volume in an electric field is given by:
In a magnetic field, the energy per unit volume is:
An electromagnetic wave has both electric and magnetic fields, so the total energy density associated with an electromagnetic wave is:
It turns out that for an electromagnetic wave, the energy associated with the electric field is equal to the energy associated with the magnetic field, so the energy density can be written in terms of just one or the other:
For an electromagnetic wave in free space, half the energy is in the electric field and half is in the magnetic field
η = +
This gives us this very compact equation for the total energy density of an electromagnetic wave…
or this one, if you prefer to state things in terms of the magnetic field instead…
This is an interesting and simple set of relations, but keep in mind that it only works for electromagnetic waves in free space. Things are different in a media and the electric and magnetic fields can have any values they want if they're static.
To relate intensity to the energy density, we first note the equality between the electric
and the magnetic energy densities ...