Reflection and absorption in matte black vs. shiny black objects

[rolaluv]

Hi,

I'm confused about reflection and absorption in materials. Is glossiness of an object determined by roughness of its surface rather than its inability to absorb visible light?

If there are dark-colored objects where one has a matte finish and the other has a glossy finish, does this mean that reflections at these surfaces (which are due to differences in refractive indices in the ray optics model) are diffuse vs. specular? In other words, will objects appear shiny if the surface is smooth and undergoes specular reflections and appear matte if reflection is diffuse?

I don't think the reflections at the surface which determines the texture of a material (shiny or matte) is due to absorption-- because if a dark object absorbs most of the incoming light, then it shouldn't reflect any light back and have a matte finish, right? So I thought every surface has some reflections because it has a different index of refraction than air, and its texture is determined by the roughness of the surface.

Am I completely mistaken on this?

Also, what is the quantum explanation of reflection?

One more point-- if my understanding of reflection is right, why are matte black objects better emitters/absorbers than shiny black objects? This contradicts my assumption where both objects absorb and reflect, but one reflects diffusely and the other reflects specularly.

 

[Stargazer19385]

The index of refraction and the angle of incidence determine the reflectance. Metals have a high index of refraction, and so are shinier.

How rough or smooth a surface is determines if it looks mirror like or matt.

How absorptive a material is determines how much light goes out the other side, not how much reflects. However, it does seem that a white car reflects more light than a black one, so I'm not completely sure about this.

As for quantumly, light raises electrons, which then fall back down and release the light. Why it goes one direction instead of another I don't know. I would think it would scatter, though it does not. But I guess depending on the frequency, the more time the photon spends on the electron, giving the dispersion we see in prisms. I doubt the speed of light in between electrons is any different for any materials, though I have heard of some materials having an index less than 1, which I guess would contradict that.

 

[sophiecentaur]

The index of refraction and the angle of incidence determine the reflectance. Metals have a high index of refraction, and so are shinier.

How rough or smooth a surface is determines if it looks mirror like or matt.

How absorptive a material is determines how much light goes out the other side, not how much reflects. However, it does seem that a white car reflects more light than a black one, so I'm not completely sure about this.

As for quantumly, light raises electrons, which then fall back down and release the light. Why it goes one direction instead of another I don't know. I would think it would scatter, though it does not. But I guess depending on the frequency, the more time the photon spends on the electron, giving the dispersion we see in prisms. I doubt the speed of light in between electrons is any different for any materials, though I have heard of some materials having an index less than 1, which I guess would contradict that.

Be careful here, because the way light interacts with solid surfaces is not as simple as the way photons interact with single atoms (what we are all taught to start with). If it were as simple as that, there would be no specular reflections and all you would get would be scattering due to absorption and random re-emission.

 

[eigenperson]

How absorptive a material is determines how much light goes out the other side, not how much reflects. However, it does seem that a white car reflects more light than a black one, so I'm not completely sure about this.

A white car does reflect more light than a black one.

Car paints use a clear binder that contains suspended pigment particles (and I think there is a clear coat on top as well). There is some specular reflection from the binder regardless of the color of the pigment; this produces the "gloss". Light that is not reflected from the binder strikes a pigment particle. If the pigment is white, the light is likely to be reflected; after some number of reflections, the light will emerge in an essentially random direction. If the particle is black, the light is likely to be absorbed.

 

[Andy Resnick]

Hi,

I'm confused about reflection and absorption in materials. Is glossiness of an object determined by roughness of its surface rather than its inability to absorb visible light?

If there are dark-colored objects where one has a matte finish and the other has a glossy finish, does this mean that reflections at these surfaces (which are due to differences in refractive indices in the ray optics model) are diffuse vs. specular? In other words, will objects appear shiny if the surface is smooth and undergoes specular reflections and appear matte if reflection is diffuse?

<snip>

Yes- 'gloss', or other appearance effects (haze, color-shifting finishes, etc) are due to surface properties, not bulk properties (of which absorption is one).

Modeling rough surface reflections can be done with ray-optics, but in general it is tedious and requires a microscopic description of the surface shape. Other models (Torrence and Sparrow, Oren-Nayar, etc.) are usually based on radiometry and use the BRDF (http://en.wikipedia.org/wiki/BRDF) to describe how light is scattered from the surface.