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Tuesday, November 16, 2010

Physical optics


Physical Optics

Polarization by Reflection:
                        Molus found that when an un-polarized light is incident on a surface of the transparent medium with polarizing angle then the reflected ray is completely plane polarized.
                        When an un-polarized light AB is incident on a glass surface with polarizing angle then the reflected light BC is completely plane polarized, which contains only dot components and the refracted ray BD is partially polarized, which contains both dot and arrow components. The polarizing angle depends upon the material of the reflecting surface. For ordinary glass the polarizing angle is 57.5°. If µ is the refractive index of reflecting surface and ip is the polarizing angle then according to Brewster’s law
                                                µ = Tan ip



Polarization of Refraction:
                        When an un-polarized light is incident on a surface of the transparent medium with polarizing angle then some part of light is reflected and some part is refracted. The refracted light is partially polarized and contains entire arrow components and few dot components.
                        When we use a pile of glass plates (number of glass plates arranged parallel to each other) the refracted light can be filtered in such a way that entire dot components are eliminated by successive reflections and only arrow components are transmitted as the light is refracted through them.
                        When an un-polarized light is incident on a surface of the first plate with polarizing angle then some of the light rays having dot components are reflected and the rest are transmitted to the second plate by refraction. The second plate again reflects the some more dot components and allows the rest. The process continuous through the successive glass plates and the out coming beam will be free of dot components and contains only the arrow components. Thus plane polarized light can be produced by refraction.
Polarization by Double Refraction:
                        When a ray of light is incident on the calcite crystal, the ray is split into two refracted rays as it enters the crystal. This phenomenon is called double refraction. This phenomenon of double refraction can be illustrated with the simple experiment. When a calcite crystal is placed on an ink dot made on a white paper and viewed through the calcite crystal, then two images of the ink dot are seen. If the crystal is rotated, it is observed that one image of the ink dot remains stationary while the other rotates about the former in direction of rotation. The stationary image is known is ordinary image and the rotating image is known is extraordinary image.
                        Consider an un-polarized light is incident on calcite crystal. Inside the crystal the ray splits up into two rays. One is called ordinary ray (o-ray) and another one is called extraordinary ray (e-ray). The ray which obeys Snell’s law is called o-ray and the ray which doesn’t obey Snell’s law is called e-ray. The o-ray contains only dot components and e-ray contains only arrow components. Thus o-ray and e-ray is linearly plane polarized. 

Applications of Interference:
1)   Used to determine the wavelength of monochromatic light.
2)   Used to determine the difference between the wavelengths of two closely – separated spectral lines.
3)   Used to determine the refractive index of liquids and gases.
4)   Used to determine the thickness of a thin transparent material.
5)   Used to test the flatness of surfaces.
6)   Used to test the reflectivity of the surfaces of lenses and prisms.
Applications of Diffraction:
1)   Used to measure the wavelengths of either monochromatic or composite radiations by using diffraction technique.
2)    Used to determine the wavelength of X-rays by using X-diffraction.
3)   Used to determine the structures of crystalline solids.
4)   Used to estimate the velocity of sound in liquids.

Applications of Polarization:
1)   Used to helpful in measuring the optical activity of crystals and liquids.
2)   Used to study the helical structure of nucleic acids.
3)   Used to study size and shape of molecules.
4)   Polarizing glasses reduce the intensity of sun light falling on the eye and prevent the damage of retina.

Difference between Fresnel and Fraunhofer Diffraction:
Fresnel Diffraction
Fraunhofer Diffraction
The source and screen are at finite distances from the slit.
The source and screen are at infinite distances from the slit.
The wave front meeting the slit is spherical or cylindrical.
The wave front meeting the slit is plane.
Diffracted light directly falls on screen.
Diffracted light is collected by means of a lens.
The centre of the diffraction pattern may be bright or dark depending upon the number of Fresnel zones.
The centre of the diffraction pattern is always bright.
No lenses are required to observe the pattern.
Two convex lenses are necessary to observe the pattern.
Theoretical treatment is complex.
Theoretical treatment is simple.