Understanding the Refractive Index in QED by Feynman

In summary, materials can have different refractive indices based on how easily they form electric or magnetic dipoles.
  • #1
jobyts
227
64
(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.)
 
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  • #2
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
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
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.
 

1. What is the refractive index in QED?

The refractive index in QED (Quantum Electrodynamics) is a measure of the speed of light in a medium compared to its speed in a vacuum. It is a fundamental property of a material and is often denoted by the symbol "n".

2. How is the refractive index calculated in QED?

The refractive index in QED is calculated by dividing the speed of light in a vacuum by the speed of light in the medium. This can be expressed as n=c/v, where c is the speed of light in a vacuum and v is the speed of light in the medium.

3. What is the significance of the refractive index in QED?

The refractive index in QED plays a crucial role in understanding how light interacts with matter. It affects the direction and speed of light as it passes through a medium, and can also determine the amount of light that is reflected or transmitted.

4. How does Feynman's work contribute to our understanding of the refractive index in QED?

Feynman's work in QED provided a theoretical framework for understanding the behavior of light and matter at the quantum level. His calculations and diagrams helped to explain the phenomenon of the refractive index and its relationship to the fundamental properties of matter.

5. Can the refractive index in QED be manipulated?

Yes, the refractive index in QED can be manipulated by changing the properties of the medium, such as its density or composition. This is the basis for many technologies, such as lenses and optical fibers, that rely on controlling the refractive index to manipulate light for various purposes.

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