Chapter 5 Polarization of Light Waves. Polarization of Light Waves Each atom produces a wave with...
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![Page 1: Chapter 5 Polarization of Light Waves. Polarization of Light Waves Each atom produces a wave with its own orientation of All directions of the electric.](https://reader035.fdocuments.in/reader035/viewer/2022062304/56649cbb5503460f9498356a/html5/thumbnails/1.jpg)
Chapter 5
Polarization of Light Waves
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Polarization of Light Waves Each atom produces a wave with its own orientation of
All directions of the electric field vector are equally possible and lie in a plane perpendicular to the direction of propagation
This is an unpolarized wave
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A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point
Polarization can be obtained from an unpolarized beam by selective absorption reflection scattering
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Polarization by Selective Absorption
The most common technique for polarizing light Uses a material that transmits waves whose electric
field vectors in the plane are parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction
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E. H. Land discovered a material that polarizes light through selective absorption He called the material Polaroid The molecules readily absorb light
whose electric field vector is parallel to their lengths and transmit light whose electric field vector is perpendicular to their lengths
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The intensity of the polarized beam transmitted through the second polarizing sheet (the analyzer) varies as
I = Io cos2 θ Io is the intensity of the polarized wave
incident on the analyzer This is known as Malus’ Law and
applies to any two polarizing materials whose transmission axes are at an angle of θ to each other
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The intensity of light transmitted through two polarizers depends on the relative orientation of their transmission axes. (a) The transmitted light has maximum intensity when the transmission axes are aligned with each other. (b) The transmitted light intensity diminishes when the transmission axes are at an angle of 450 with each other. (c) The transmitted light intensity is a minimum when the transmission axes are at right angles to each other.
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Polarization by Reflection When an unpolarized light beam is reflected
from a surface, the reflected light is Completely polarized Partially polarized Unpolarized
It depends on the angle of incidence If the angle is 0° or 90°, the reflected beam is
unpolarized For angles between this, there is some degree
of polarization For one particular angle, the beam is
completely polarized
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The angle of incidence for which the reflected beam is completely polarized is called the polarizing angle, θp
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θp + 90o + θp = 180°, so that θ = 90° - θp .
Using Snell’s law and taking n1 = 1.00 and n2 = n, we have
Because sin θ2 = sin(90° - θp) = cos θp , the
expression for n can be written as
θp may also be called Brewster’s Angle
sintan
cosp
pp
n
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Polarization by Double Refraction When light travels through an amorphous
material, such as glass, it travels with a speed that is the same in all directions.
That is, glass has a single index of refraction. In certain crystalline materials, however, such as calcite and quartz, the speed of light is not the same in all directions. Such materials are characterized by two indices of refraction. Hence, they are often referred to as double-refracting or birefringent materials.
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When unpolarized light enters a calcite crystal, it splits into two plane-polarized rays that travel with different speeds, corresponding to two angles of refraction
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One ray, called the ordinary (O) ray, is characterized by an index of refraction, nO that is the same in all directions.
The second plane-polarized ray, called the extraordinary (E) ray, travels with different speeds in different directions and have an index of refraction, nE, that varies with the direction of propagation.
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there is one direction, called the optic axis, along which the ordinary and extraordinary rays have the same speed, corresponding to the direction for which
nO = nE . The difference in speed for the two rays is a
maximum in the direction perpendicular to the optic axis.
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Polarization by Scattering When light is incident on a system of
particles, the electrons in the medium can absorb and reradiate part of the light This process is called scattering
An example of scattering is the sunlight reaching an observer on the earth becoming polarized
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The horizontal part of the electric field vector in the incident wave causes the charges to vibrate horizontally
The vertical part of the vector simultaneously causes them to vibrate vertically
Horizontally and vertically polarized waves are emitted
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Optical Activity Certain materials display the property of
optical activity A substance is optically active if it
rotates the plane of polarization of transmitted light
Optical activity occurs in a material because of an asymmetry in the shape of its constituent materials
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The angle through which the light is rotated by a specific material depends on the length of the sample and on the concentration if the substance is in solution.
One optically active material is a solution of common sugar dextrose.
A standard method for determining the concentration sugar solutions is to measure the rotation produced by a fixed length of the solution.
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photoelastistity materials, such as glass
and plastic, become optically active when stressed
If the plastic is stressed, regions of greatest stress rotate the polarized light through the largest angles.
a series of bright and dark bands is observed in the transmitted light;