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Advanced Physics Topics like quantum mechanics and relativity have revolutionized our understanding of the universe.
What do you Mean by Optic Axis? What are Uniaxial and Biaxial Crystals?
Polarisation By Refraction
When an ordinary (unpolarised) light is incident on the upper surface of a parallel-faced glass plate at the polarising angle, the reflected light is completely plane polarised but its intensity is very low. Major portion (about 85%) of the incident ray is refracted and only a very small portion (15%) is reflected. The refracted ray is also partly polarised. The two planes of polarisation of completely polarised reflected ray and partly polarised refracted ray are at right angles to each other. So it is not possible to get a strong reflected beam of polarised light with the help of a single plate.
To overcome this defect, a number of plane parallel glass plates are placed parallel to each other and an unpolarised light is allowed to fall on the first plate at the polarising angle. Due to successive reflections, strong beams of polarised reflected light will be obtained. Ultimately two plane polarised light will be separated-one reflected polarised light with vibrations perpendicular to the plane of the paper i.e., the plane of incidence and another refracted polarised light with vibrations in the plane of the paper [Fig.].
Double Refraction Or Birefringence
In 1669 Rasmus Bariholin discovered that when an ordinary ray of light is incident perpendicularly on the surface of a calcite crystal, it splits into two rays due to refraction. One of the refracted rays obeys the laws of refraction and is called the ordinary ray or O-ray. The other refracted ray does not obey these laws; hence it is called the extraordinary ray or E-ray. Both of these rays are plane polarised in mutually perpendicular planes.
The phenomenon by virtue of which an unpolarised ordinary ray, on entering a crystalline substance, splits up into two rays i.e., O-ray and E-ray, is called double refraction or birefringence. The crystals in which double refraction takes place are called double refracting crystals, Examples of such crystals are calcite, quartz, tourmaline, etc.
In Fig. the splitting up of an unpolarised incident light PQ into O-ray and E-ray in a calcite crystal has been shown. Here QR is the ordinary ray (O-ray) and QS is the extraordinary ray (E-ray). O-ray advances through the crystal at same velocity in all directions, but E-ray advances at different velocities in different directions.
If the crystal is rotated slowly about PQ as axis, it will be observed that one of the images of the source remains stationary while the other revolves about the first. The image which remains stationary is produced due to ordinary ray while the other one is produced due to extraordinary ray.
Double refracting crystals are classified into two types-
- positive crystals and
- negative crystals.
i) Positive crystal: The double refracting crystals, where velocity of O -ray (VO) is more than velocity of E -ray (VE) i.e., VO > VE, are called positive crystals. Here refractive index of the crystal for O -ray (µO) becomes less than refractive index of the crystal for E -ray (µE), i.e., µO < µE.
Example: quartz (SiO2, here µ0 = 1.544, µE = 1.553); ice (H2O, here µO = 1.309, µE = 1.313) etc.
ii) Negative crystal: The double refracting crystals, where velocity of O -ray (VO), is less than velocity of E – ray (VE), i.e., VO < VE, are called negative crystals. Here refractive index of the crystal for O -ray (µO), becomes higher than the refractive index of the crystal for E -ray (µE) i.e., µO > µE.
Example: tourmaline (a complex silicate compound, here µO = 1.669 and µE = 1.638), calcite (CaCO3, here µO = 1.658 and µE = 1.486) etc.
Optic axis: If a ray is incident on a crystal in such a direction that double refraction does not take place inside the crystal, then that direction is called the optic axis. Hence O -ray and E -ray both advance at same velocity in the same direction along the optic axis.
Uniaxial and biaxial crystals: The crystals which have only one optic axis are called uniaxial crystals.
Example: calcite (CaCO3), quartz (SiO2) etc.
The crystals which have two optic axes are called biaxial crystals.
Example: aregonite, copper sulphate etc.