Saturday, November 3, 2018

optics - Why can you have shiny black objects?


Knowing black is supposed to be the "color" (I don't want to get into the color/hue/shade debate, please) that absorbs light. how does one manage to have shiny black surfaces? I know about "gloss black" versus "matte black" finishes, but shouldn't the light passing through the gloss (if they didn't pass through the gloss, you wouldn't see the black, right?) be absorbed by the underlying black object? Then there are black gemstones like jet and opal.


How do black objects shine?



Answer



At any surface (at least one which has a different index of refraction from air) some light is reflected, depending on the angle at which the light hits the surface and the polarization of the light; the Fresnel equations will tell you what fraction of your light is reflected and what fraction is transmitted. When you see a black object "shine", you are seeing the reflected light. But since the object is black, all the "transmitted" light is simply absorbed.


The difference between a matte black and a gloss black finish is one of index of refraction, I guess, and possibly of rough/smoothness.



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