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Fresnel's Equations

In the Snell's law page we figured out the angle the light will be reflected and transmitted from an interface. What we haven't figured out yet is the amount of light that will be reflected or transmitted. Fresnel's equations calculate the fraction of light reflected or transmitted using the index of refraction, discussed in the Snell's law page, and the angles of incidence and refraction.

Fresnel's Equations
(You can click on the image above for a really nice, interactive, page on Fresnel's equations at the hyperphysics site.)

Since the sum of R and T is equal to one, R and T must both be fractions of the light that came in. So if we know the intensity of the light incident upon the surface we can calculate the fraction of the incident intensity that is reflected or transmitted. 

We care about the fraction of light transmitted or reflected because light carries momentum. How light can have momentum but not mass is exhaustingly derived in the next page.