Non Life Sciences => Chemistry => Topic started by: tintinxx on 27/11/2007 00:46:38
Title: Briefringence?
Post by: tintinxx on 27/11/2007 00:46:38
I understand that birefringent material has two refractive indices. How does the direction of light's electric field comes into play? [???]
Title: Briefringence?
Post by: lyner on 27/11/2007 14:29:56
The refractive index of a material is a measure of how easily an electric field can make the electrons move from side to side. This affects how quickly the light can propagate through the medium - getting the electrons moving delays the passage of the wave and it goes at less than c. Most substances are isotropic - the refractive index is the same on all axes so the speed is the same for all polarisations.. Some crystals have a large difference in the way the electrons move along one axis and another. If the E field of the light is aligned one way it will travel faster than if it is aligned the other way. (The direction of the E field is what we call the polarisation.)
There are two visible effects. The two different refractive indices will split a beam of light into two paths when it enters at an angle. You will see two images through a crystal of calcite, for instance, if you get the angles right. The other effect is the phase difference between the vertical and horizontal polarised components of a light beam. This effect can be used in a thin sheet to produce a 'polariser', which will only let one polarization through - as in polaroid specs. LCDs use polarisers, too.
Title: Briefringence?
Post by: tintinxx on 27/11/2007 20:10:42