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I was surprised to see that the Fresnel equations for reflection depends on refractive index and angle of incidence, but they do not depend on frequency. Why is this case? Are they restricted to regions that do not show frequency dependence?

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JHN
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    For real materials the index of refraction is frequency dependent, which makes the Fresnel equations frequency dependent. – CuriousOne Apr 21 '16 at 06:35

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The Fresnel equations are derived by matching the electric and magnetic fields of the incident, reflected and transmitted waves at the interface. In this process only the instantaneous value of the fields is used not their rate of change with time. This means the frequency of the wave simply doesn't enter the calculation.

However, as a comment notes, the refractive index is frequency dependent and this means the reflectivity is also frequency dependent.

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John Rennie
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