Question about how we see materials and absorption

[adgalati]

In griffith's introduction to electrodynamics, he goes over dispersion/absorption using the oscillating electron model. I understand the theory but I am confused about how it relates to how materials appear to us. He says that the material will be practically opaque in the frequency range with maximum absorption due to the large dissipation of energy from resonance. Then he says transparent materials have their significant resonances in the ultraviolet, which makes sense to me as to why we can see through them, they don't really absorb any visible light so it passes right through them. So from this, my understanding is that a materials color depends where on the spectrum their resonance frequencies lie. My confusion comes in when I try to think of a black material. I don't see any color coming from it, yet I can't see through it. Is it that the resonance frequencies are so scattered across the spectrum that it doesn't show any distinct color?

 

[Simon Bridge]

Welcome to PF;
We can see through materials that do not absorb any visible wavelengths.
Black coloured materials absorb all visible wavelengths they meet, well done.

 

[adgalati]

Thank You! However now I am confused about white materials. I was given to believe that white light was all of the visible wavelengths, so I had assumed that they would be absorbing all wavelengths as well. Am I correct in that reasoning or is there another explanation?

 

[Simon Bridge]

The model you are looking at is too simple to account for all the ways materials may be coloured.

i.e. Not all light is absorbed - the rest may be transmitted, reflected, or scattered.
So far you have been considering the colour of a material on transmission - so you are thinking more of a transparent material with a coloured tint.
If the material blocks all light, i.e. by absorbing it all, then it is "dark" ... we are looking at the shadow side.
We wouldn't really think of it as being coloured black.

Very few materials are white on transmission, we'd think of a white translucence rather than a tint.
White translucence is typically due to multiple scattering before the light can pass through the material - i.e. clouds, or frosted glass.

When you see the colour of the brightly lit surface, you are looking at scattered light (this is usually called the diffuse colour).
Absorption has the strongest effect on this - so the colour that gets scattered is pretty much determined by the wavelengths absorbed.

Sometimes you will see a shimmery colour that is different from the diffuse colour - this is called the specular colour, results from reflection of the outer surface.
An object that reflects everywhere in the visible spectrum is what we call called a mirror... though, in physics, we only care that it reflects the wavelengths of interest.

There are more complicated effects, like iridescence ... in that case there is often interference between different layers of material.

 

[adgalati]

I see. I had assumed that, as in many other areas of physics, one model cannot explain all phenomena. Thank you!

 

[Simon Bridge]

There is this idea in physics that there is one theory that will explain/model all phenomena ... that is to say: that all other models can be derived from. This is known as the Grand Unified Theory, and we are not even close to finding one, though some people have high hopes for string theory.

At the level you are working at - it's more that you are introduced to approximate models to get you used to the way of thinking and the maths before anyone tells you the real, inclusive, theory ... in this case, it's quantum mechanical.

 

[adgalati]

Yes, I'm in my last semester of a BS in physics so I still have a lot to learn! I am taking an optics course now so hopefully I'll be able to get a better general idea about this subject

 

[Simon Bridge]

Enjoy.