Questions about EM Waves Refraction

I've 2 questions, Please help !

1. at an interface between two dielectric media how to explain the condition Ei+Er=Et. (E: vector)
2. in some cases the transmission factor τ=2n1/(n1+n2) > 1 (Amplification?!) which means that the amplitude of the transmitted (refracted) wave is greater than that of the incident one (the source) though there is a reflected wave.
Thanks

Re: EM Waves Refraction

Hi,
The first condition comes from the continuity of the tangential electric field at interface between two media. The continuity is one of the four important boundary conditions for electromagnetic wave which states that the tangential component of the electric field at the interface must continue i.e. tangential component of the total electric field one side of the boundary must be equal to the same at the other side. So follows the condition

About the second query: At the interface the reflected wave is not in phase with the incident wave and so is the transmitted wave. Although the total em energy of the incident wave must be equal to the enrrgy of the transmitted and the reflected wave. So althought the field amplitude of the transmitted wave can exceed that of the incident wave the energy of the transmitted wave can never.
Another thing is that the expression for the transmittance is what you mentioned only when the electric field of the em wave is parallel to the interface i.e. for free wave the wave has to fall perpendicularly.