# Dielectrics, Varying Reflectivity, and Polarization

## Main Question or Discussion Point

I am very curious as to why dielectrics such as water have a varying level of reflectivity depending on what angle they are viewed at. For a lake of water, what angle yields 100% (or near 100%) reflectivity? Is this angle the same as the Brewster angle?

Also, why exactly is it that metals do not polarize light upon reflection while dielectrics do?

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Matterwave
Gold Member
Snell's law for the transmitted wave gives you n1sin(theta1)=n2sin(theta2), if the conditions are correct, you can have the situation where theta2>=90degrees. In this case, there is no transmitted wave, the "transmitted wave" just propagates along the surface of the water (the wave-vector of this wave becomes imaginary and so the fields do not propagate into the water, but rather, the fields decay).

This is called "total internal reflection".

Brewster's angle is kind of like the opposite. In that case, it is the REFLECTED wave which disappears (rather than the transmitted one). But Brewster's angle only affects waves of one type of polarization, whereas total internal reflection does not care about the polarization of the waves.

The reflectivity of a dielectric surface strongly depends on the angle of incidence, as governed by the Fresnel equations. The Fresnel equations are derived by applying boundary conditions to the waves at the surface. You can think of a wave striking and entering a surface as creating a train of little radiating dipole antennas in the material, situated perpendicular to the direction the wave is traveling. The resultant wave is the sum of all the re-radiated waves. Antennas radiate strongest out their sides and weakest out the top (along their axis), so that lower viewing angles will see stronger reflection.

All materials become 100% reflective as our viewing angle approaches 90 degrees from the normal, i.e. at grazing angles. http://faculty.uml.edu/cbaird/95.658%282011%29/Lecture1.pdf" [Broken].

Dielectrics only polarize light near Brewster's angle.

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