Why do some materials reflect and others absorb light?

[jostpuur]

If the question was that why are some materials opaque and why some are transparent, the answer would be found in the energy bands. If energy bands allow such transitions for electrons, that they can absorb photon, then they will absorb it, and the photon does not go through the material. If electrons instead cannot absorb a photon, then the photon will go through the material.

Was that right?

Well then the next question is that why are some materials reflective and some absorptive? Can it be explained with the energy bands?

 

[ZapperZ]

What are "energy belts"?

There is an elementary explanation for photon transport in materials in the FAQ thread inside the General Physics forum. Maybe you should start with that first and see if you have any further question.

Zz.

 

[jostpuur]

I meant "energy band".

If you translate "band" into Finnish and then back to English, it easily becomes a "belt".

I don't believe that the FAQ answers my question. It contains something about absorption, but not about reflection.

 

[ZapperZ]

I meant "energy band".

If you translate "band" into Finnish and then back to English, it easily becomes a "belt".

I don't believe that the FAQ answers my question. It contains something about absorption, but not about reflection.

I did say it is a "start".

Reflection is a bit more complicated, because it depends on the material. Most mirrors that you see are metals, so this clearly provides a clue on the process of reflection in the visible range. The free electrons form what is often-called the "plasmon" collective state, which is a surface phenomenon that is responsible for most reflection of such metallic surface.

Zz.

 

[Naty1]

Sounds like you are talking light..electromagnetic radiation, right?...not ionized particle radiation...different mechanisms are involved.

I don't know a simple answer...but there are several interpretations here:

http://en.wikipedia.org/wiki/Reflection_of_light

Maybe somehting here: http://en.wikipedia.org/wiki/Optical_coating

including antireflective coatings

let us know what you find...

 

[DrDu]

A material is absorptive if its dielectric constant has an imaginary part.
If epsilon_r is not equal to 1, it is also reflective, at least to some extent.
This makes strongly absorbing substances also highly reflective as can be seen e.g. in crystalls of pure dyes.

The dielectric constant can be calculated in the band approximation. The classical
reference being S. Adler, Quantum Theory of the Dielectric Constant in Real Solids, Phys. Rev. 126:413-20 (1962)