What is the effect of frequency of light on reflection from a surface? Suppose we have a light source with a frequency $f$. The light source shines on a surface with known properties (eg. texture, material, etc.) for example a sheet of unpolished steel. What is the effect of frequency (increasing or decreasing it) on the intensity of reflection from the surface? Does it have any effect at all? What about the reflection's angle?
 A: In optics, the reflectivity is calculated from angle of incidence and the complex refractive index according to Fresnel's equations. You may have a look here https://en.wikipedia.org/wiki/Fresnel_equations. The complex refractive angle $n = \sqrt{\epsilon_r \mu_r}$ is based on the relative permittivity $\epsilon_r$ (for magnetic materials also $\mu_r$) and therefore frequency-dependent (https://en.wikipedia.org/wiki/Permittivity, in that link there is a section "complex permittivity" which I found quite useful).
A: Sorry for my poor english. French is may native language.
The reflected wave can be considered as the response of the medium to the incident wave which is the excitation. If the incident wave is sinusoidal, one seeks the forced response to a sinusoidal excitation.
Most often the medium is modeled as linear (polarization proportional to the excitation ....). It is a general property of linear media that the forced sinusoidal response is of the same frequency as the excitation. (Conversely, the appearance of harmonics is a criterion of non-linearity)
A: The reflection coefficient is indeed a function of frequency.  You know this but probably did not think about it.  A red surface reflects red light more than blue,  maybe.  For each material and surface quality there is a specific dependence on frequency. The metals like silver or aluminum may reflect all visible frequencies about the same (but not UV light). Gold and copper reflect more yellow-red. The angle of reflection though is not a function of frequency.
