Suppose you measure the reflection value, at normal incidence, of a dialectric surface. Let's call the value x.

Then the index of refraction (IOR) of this material is easily calculated from:

(1 + sqrt(x)) / (1 - sqrt(x))

This is easy to derive from the inverse of the Fresnel equation with theta = 0.

There is no need to measure the angle of refraction or amount of refraction. The reflection value, refraction value, and refraction angle are all driven by the same equations. Solve for one, and you've solved for all.

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