I'm working on Fresnel equation for calculation of reflection of a light (532 nm) on Iron.
I've got a question: Is metals refractive index always a real number or it can be a complex number?
Metals refractive index is always complex number (and not only for metals). Imagine part shows the extinction coefficient $k$ - absorption in a material.
Real and imagine part isn't connected.
P.S. For engineering calculations real part sometimes is less than 1.
Theoretically even for Fresnel reflection in dielectric we must use full formula with complex part:
for dielectrics (glasses) in visual diapason $k<<1$ so we don't use it, but u got metal ;-)
The above answer is correct for certain wavelength, for metal, especially for longer wavelength, the refractinve index is complex number. But when wavelength become shorter, to X ray wavelength or even shorter, metal bacome transparent, and might finally have a real refractinve index. See the database: https://refractiveindex.info/