Why do some materials reflect and others absorb light?

In summary, if the question was about why some materials are opaque and some are transparent, it is because they have different energy bands which allow for the transition of electrons. If energy bands allow for such transitions, then the electron will absorb the photon and the photon will not go through the material. If electrons instead cannot absorb a photon, then the photon will go through the material. Reflection is more complicated than absorption because it depends on the material. Most mirrors are made of metal, which explains how reflection works in the visible range.
  • #1
jostpuur
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19
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?
 
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  • #2
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.
 
  • #3
I meant "energy band".

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

I don't believe that the FAQ answers my question. It contains something about absorption, but not about reflection.
 
  • #4
jostpuur said:
I meant "energy band".

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

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.
 
  • #6
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)
 

1. What is the difference between reflection and absorption?

Reflection refers to the bouncing back of light or other electromagnetic radiation when it hits a surface. Absorption, on the other hand, occurs when the light or radiation is absorbed by the surface and converted into heat or another form of energy.

2. How does the color of an object relate to its reflection and absorption?

The color of an object is determined by the wavelengths of light that are reflected or absorbed by its surface. Objects that appear white reflect all wavelengths of light, while objects that appear black absorb all wavelengths.

3. What causes reflection and absorption to occur?

Reflection occurs due to the interaction of light with the surface of an object. The smoothness and angle of the surface determine how much light is reflected. Absorption occurs when the energy of light is absorbed by the atoms and molecules in the surface of an object.

4. How do different materials affect reflection and absorption?

Materials with smooth and shiny surfaces, such as glass or metal, tend to reflect more light than rough and dull surfaces, such as paper or fabric. Different materials also have different levels of absorption, depending on their composition and structure.

5. How does the angle of incidence affect reflection and absorption?

The angle at which light hits a surface, known as the angle of incidence, affects the amount of reflection and absorption that occurs. When light hits a surface at a perpendicular angle, more of it is reflected. As the angle of incidence increases, more light is absorbed and less is reflected.

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