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## O-ray vs E-ray | Difference between O-ray and E-ray

This page compares O-ray vs E-ray and mentions difference between O-ray and E-ray. The O-ray stands for Ordinary ray and E-ray stands for Extraordinary ray.

Introduction:
Double refraction or Birefringence is special optical property which is observed in distinct type of anisotropic crystals such as calcite, quartz, KDP, rutile etc. Both O-ray and E-ray are associated with this. When single beam of unpolarized light is incident on such anisotropic crystal (e.g. Calcite), light is splitted into two beams (viz. o-ray and e-ray) at the crystal surface as shown in the figure-1. Both O-ray and E-ray propagate in two different directions with different velocities. They have mutually orthogonal planes of polarization.

### O-Ray | Ordinary ray

• As shown, plane of polarization of o-ray is perpendicular to the plane of figure.
• It is known as ordinary ray.
• It follows Snell's law of refraction.
• The velocity of o-ray i.e. vo is c/no.

### E-Ray | Extraordinary ray

• As shown, plane of polarization of o-ray is in the same plane as of figure.
• It is known as Extraordinary ray.
• It does not follow Snell's law of refraction.
• The velocity of e-ray i.e. ve is c/ne.

The velocities of o-ray and e-ray are different in all the directions except in one particular direction known as optic axis. In this optic-axis, both o-ray and e-ray travel with same velocities. The effective refractive indices no and ne are known as principal refractive indices of the crystal.