Refractive index

  • Thread starter jobyts
  • Start date
  • #1
201
44
(I'm still reading the QED book by Feynman...)

What property of the material causes a specific refractive index for a particular medium? (in other words, from the FAQ section by ZapperZ, "So the lattice does not absorb this photon and it is re-emitted but with a very slight delay.". How is the delay different between different materials.)
 

Answers and Replies

  • #2
468
4
Classically, it comes down to how easily a material can form electric or magnetic dipoles. In a dielectric, for example, when an electric field is applied, the bound electrons will move further away from their host nuclei, forming dipoles which will in turn reinforce that field. For most materials the polarization of the material is proportional to the applied field, and the constant of proportionality is wrapped up into the electric permittivity. Likewise, applied magnetic fields will often induce a proportional response in the magnetic polarization: this is wrapped up into a constant known as the permeability. Together, the permittivity and permeability determine the speed of field propagation through the material, which in turn gives the index.
 
  • #3
Claude Bile
Science Advisor
1,471
19
Note too that the refractive index can be influenced by external factors as well such as applied E-field and temperature. The things Manchot mentions don't just affect the refractive index, but also how the refractive index changes as a function of these external factors.

Claude.
 
  • #4
468
4
Yeah, and you should also note that the refractive index may not even be well-defined for some situations. For example, in some materials like crystals or polymers, the polarization responds to an applied field anisotropically, meaning that certain directions are preferred over others. In these cases, the permittivity and index must be described by matrices. In other materials, the polarization responds to an applied field non-linearly, in which case the index is a function of field amplitude. As a matter of fact, all materials are essentially nonlinear for large fields: when a field larger than the breakdown field strength is applied, a dielectric becomes conductive, and all of this goes out the window.
 

Related Threads on Refractive index

  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
13
Views
10K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
14
Views
2K
  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
1
Views
2K
  • Last Post
Replies
1
Views
528
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
9
Views
2K
Top