Unoxidized aluminum has a shiny appearance like typical metals. However in air aluminum rapidly oxidizes, giving it a more diffuse appearance (or bidirectional reflectance distribution function). Which (if any) of the following would be a more accurate description for the mechanism driving this change:
- The oxidation introduces surface roughness at the scale of light's wavelength, causing what we can think of as lots of specular microfacets that scatter the light in different directions.
- The oxidation introduces subsurface irregularities/"particles" (at or smaller than the scale of light's wavelength). Incoming light is then refracted through the metal's surface into the metal, scattered internally by these subsurface irregularities, before refracting back out through the surface.
My uneducated impression is that:
(1) seems unlikely because of how much smaller atoms and molecules are than light's wavelength
(2) seems falsified by the absence of even faint specular highlights on the aluminum that is typical of dielectrics with subsurface scattering, such as glossy opaque plastic.