Double Refraction: How It Works

[Ezio3.1415]

How does double refraction work?

 

[sophiecentaur]

You have presumably looked it (birefringence) up already and you need an explanation of the actual mechanism rather than a simple definition.
It occurs in a medium that is not isotropic - the refractive index along one axis is different from the refractive index along another axis. Refractive index is affected by how the molecules polarise in the presence of an electric field, which, in turn, affects the speed of the wave propagation through the medium. If the medium is a crystal or some 'stressed' amorphous solids, the electrons may be a bit more easily displaced in one direction than in another so the molecules can distort by a different amount. Calcite is the classic example of such a crystal. Light of one plane of linear polarisation travels faster than light in an orthogonal plane and so unpolarised light is split into two beams when it travels through obliquely - the random selection of different polarisation vectors from all the different waves passing through are all resolved into ('H' and 'V') components, corresponding to the two planes in the crystal, each one being refracted by a different amount.

 

[Ezio3.1415]

Thank you for understanding what I meant and answering with a good explanation...

Please tell me if I am right... First there are alternations of E in every direction...When it goes through Calcite the calcite polarizes it... We take two planes perpendicular to each other(principal plane and the other perpendicular to it) and find out the vertical and horizontal component... And as calcite not isotropic regarding the structure of molecules thus the refractive index is not same in the 2 planes... Thus the ray splits into two rays having different velocity and goes different direction...

btw: Why don't we talk about magnetic field alternation when talking about polarization and regarding phenomena?

 

[sophiecentaur]

That's quite right enough for starters. We only consider the E field because the fields are at right angles and the H field doesn't need to be described. The ratio may be different for different media though.

 

[Ezio3.1415]

Thank you...

Another question... In single slit fraunhofer diffraction, only the light which goes near the slit is diffracted... If it goes just through the middle it will not be... If the slit is really small would light be diffracted even if it goes through middle?

And do all light waves that go near the slit diffract? Does all light that goes through the middle go undiffracted and contribute to central maximum? What would be the classic and quantum interpretation of this question?