Monday, February 18, 2008

POLARISATION OF LIGHT

Polarisation
The experiments on interference and diffraction have shown that light is a form of wave motion. But these effects, do not give an idea about the type of wave motion. The phenomenon of polarization has helped to establish that light waves are transverse in nature.
Production of plane polarized light
Some of the methods for the production of plane polarized light are
(1) Reflection,
(2) Refraction,
(3) Double refraction,
(4) Selective absorption in crystals and
(5) Scattering.
Polarisation by reflection
An ordinary beam of light on reflection from a transparent medium like glass, water etc. becomes partially polarized. The degree of polarization increases as the angle of incidence increased. At a particular value of angle of incidence, the reflected beam becomes completely polarized. This angle of incidence is called the polarizing angle (ip) for that medium.
Brewster’s law
Refractive index of the material medium is equal to tangent of the angle of polarization. This is called Brewster’s law.
i.e, µ = tan ip = sin ip/cosip ----- (1)
From Snell’s law, µ = sin ip/sin rp ----- (2)
Comparing (1) and (2), sin rp = cosip = sin (90- ip)
\rp = 90 – ip
Or, ip+rp = 900
i.e., when light is incident at the polarizing angle, the reflected and refracted rays are perpendicular to each other.
Double refraction (Birefringence)
It is seen that when a beam of ordinary monochromatic light is allowed to pass through a calcite or quartz crystal, we get two refracted beams instead of the usual one as in the case of glass. This phenomenon is called double refraction or birefringence.
When an illuminated pin hole is observed through a calcite crystal, two images are seen. If the crystal is rotated about the incident ray as axis, it is found that one of the images remains stationary while the other rotates round the stationary image. The stationary image is known as ordinary image while the other is known as extra ordinary image. The ray giving rise to the ordinary image is called the ordinary ray and the ray giving rise to the extra ordinary image is called extra ordinary ray.
The ordinary ray obeys laws of refraction while the extra ordinary ray does not. The refractive index of a doubly refracting crystal is a constant with respect to the ordinary ray. The refractive index of the crystal with respect to the extraordinary ray varies with the angle of incidence.
It has been found that both ordinary and extra ordinary rays are plane polarized with their vibrations being at right angles to each other. Ordinary ray has vibration perpendicular to the principal section and extra ordinary ray has vibration parallel to the principal section.
For a calcite crystal, rE is greater than ro. Therefore, µo is greater than µE.
µE=1.486
µo=1.658
For a quartz crystal, rOis greater than rE. Therefore, µE is greater than µO.
µE=1.553
µo=1.544
Calcite crystal
Calcite crystals are generally used in polarisation experiments.It is transparent to visible and ultraviolet light.Chemically it is CaCO3 and assumes rhomohedron crystal structure

No comments: