DOUBLE REFRACTION

When ordinary light is allowed to pass through certain crystal like calcite or quartz it is split up into two refracted rays namely ordinary wave or “ O “ray and extraordinary wave or “e” ray . this phenomenon is called double refraction and the crystals showing this property called double refracted crystals .
double refraction
Both O ray and e ray are plane polarized. But their plane of polarization mutually perpendicular to each other. O ray obeys the law of refraction where as e ray doesnot velocity of o is same throughout the crystal since the refractive index remains the same where as the velocity of e ray is different in different directions due to the variations in refractive index . along direction of optic axis ( a direction passing through blunt corner ) both o ray and e ray travel with same velocity along the same path.
Lecture Notes – Optics 3: Double Refraction, Polarized Light
• Experiment: observations with optical calcite.
• Light passing through a calcite crystal is split into two rays. This process, first reported by
Erasmus Bartholinus in 1669, is called double refraction. The two rays of light are each plane
polarized by the calcite such that the planes of polarization are mutually perpendicular. For
normal incidence (a Snell’s law angle of 0°), the two planes of polarization are also perpendicular to the plane of incidence.
• For normal incidence (a 0° angle of incidence), Snell’s law predicts that the angle of refraction will be 0°. In the case of double refraction of a normally incident ray of light, at least one of the two rays must violate Snell’s Law as we know it. For calcite, one of the two rays does indeed obey Snell’s Law; this ray is called the ordinary ray (or O-ray). The other ray (and any ray that does not obey Snell’s Law) is an extraordinary ray (or E-ray).
• For ordinary rays the vibration direction, indicated by the electric vectors in our illustrations, is perpendicular to the ray path. For extraordinary rays, the vibration direction is not perpendicular to the ray path. The direction perpendicular to the vibration direction is called the wave normal. Although Snell’s Law is not satisfied by the ray path for extraordinary rays, it is satisfied by the wave normals of extraordinary rays. In other words, the wave normal direction for the refracted ray is related to the wave normal direction for the incident ray by Snell’s Law.

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