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I am getting into the electromagnetic waves section in my optics class and in some of these derivations, they are using expressions that I cannot remember how they were derived or the context of them. My book from my E&M class I took awhile back is currently at my house and I am at my school so I can not look it up. The expressions I am talking about are..

[tex]E=cB[/tex]

Where E and B are the magnitudes of the electric and magnetic fields respectively.

Energy Density in the Electric field

[tex]u_{E}=\frac{1}{2}ε_0E^2[/tex]

Energy Density in the Magnetic field

[tex]u_{B}=\frac{1}{2} \frac{1}{μ_0}B^2[/tex]

Can anyone show where these expressions come from? It would help my understanding a lot! Thanks!