Download Polarization of Light

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Unit 10.1 Polarization of Light
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Physics Tool box
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Longitudinal waves: In a longitudinal wave the particle displacement is
parallel to the direction of wave propagation.
Transversal waves: In a transverse wave the particle displacement is
perpendicular to the direction of wave propagation
Polarization of light can be achieved in the following ways: double refraction,
reflection, scattering, and a polarization filter.
Polarization provided the proof that light is a transverse wave.
Scattering occurs when light from the Sun passes through our atmosphere
and encounters small particles that scatter light.
Polarization filters have many uses, including glare reduction, stress analysis,
and photography.
Young’s experiment demonstrated that wave properties of light, but it did not give any
indication if the light waves are transverse of longitudinal. If we assume that vibrations are
transverse and that they vibrate in all directions, perpendicular to the direction in which the
light is travelling.
Then if the transverse wave passed through a filter that allowed vibrations to occur in only
one direction. Then the wave would be polarized in that direction (plane-polarized)
The vertically polarized waves below pass through a vertical slit but are stopped by a
horizontal polarizing slit.
Thus when light is passed through a polarization filter, the light waves are polarized in one
plane (only light vibrating in that direction can pass through the slit). Now if you have
another polarizer (called an analyzer) that is perpendicular to the first, then almost all the
light will be blocked (absorbed, reflected).
SPH3UW/SPH4UI
Unit 10.1 Polarization of Light
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If light was a longitudinal wave, then polarizing filters would not stop them. Thus light is a
transverse wave (or acts like transverse waves).
Polarization can be achieved in 4 ways:
1)
2)
3)
4)
Double refraction
Reflection
Scattering
Polarization Filter
Materials such as glass and Lucite, which become doubly refractive when subjected to
mechanical stress, are said to have photo-elasticity. When you place a photo-elastic material
between two polarizers, the stress patterns are revealed.
Polarized Light
An ordinary light beam is composed of a large number of waves that where emitted by the
atoms of the light source. Each atom produces a light wave having some particular orientation
of the electric field vector E, corresponding to the direction of atomic vibration (recall that the
magnetic, B, field vibrates perpendicular to this direction).
The direction of the polarization of each individual light wave is
defined to be the direction in which the electric field (E) is vibrating.
In the Figure above where the light wave is travelling in the x
direction, the polarization lies along the y axis. However, an
individual electromagnetic wave could have its E vector in the yz
plane, making any possible angle with the y axis. Because all
directions of vibration from a wave source are possible, the
resultant electromagnetic wave is a superposition of waves
vibrating in many different directions. The result is an unpolarized
light beam represented in the Figure to the right.
The direction of wave propagation in this figure is perpendicular (out or in) to the page. The
arrows show a few possible directions of the electric field vectors for the individual waves
making up the resultant beam. At any given point and at some instant of time, all these
individual electric field vectors add to give one resultant electric field vector.
SPH3UW/SPH4UI
Unit 10.1 Polarization of Light
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A light wave is said to be linearly polarized if the resultant electric field E vibrates in
the same direction at all times at a particular point. The plane formed by E and the
direction of propagation is called the plane of polarization of the wave. It is possible
to obtain a linearly polarized beam from an unpolarized beam by removing all
waves from the beam except those whose electric field vectors oscillate in a single
plane. This process will provide us with polarized light that now has a single axis of
polarization.
Malus’s Law
Malus' law, which is named after Etienne-Louis Malus, says that when a perfect polarizer is placed in a
polarized beam of light, the intensity, I, of the light that passes through is given by:
I  I o cos2  
Note: If unpolarized light passes through a polarizer,
then ½ the intensity passes through.
Where Io represents the initial intensity and  represents the angle between the light's initial polarization
direction and the axis of the polarizer.
Intensity= I o
Intensity=
1
Io
2
1

I o cos 2   
2

Intensity = 
SPH3UW/SPH4UI
Unit 10.1 Polarization of Light
Example
Determine the intensity of I3
I1
I2
Solution
I1: Since random light passes through a polarizer we have: I1 
I2: Apply Malus’s Law : I 2  I1 cos 2  45 
1
 I 0 cos 2  45 
2
1 1
 I0
2 2
1
 I0
4
I3: Apply Malus’s Law: I 3  I 2 cos 2  45 
1
 I 0 cos 2  45 
4
1 1
 I0
4 2
1
 I0
8
The final intensity is
1
of the original intensity.
8
1
I0
2
Difference between 00
and 450
Difference between 900
and 450
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SPH3UW/SPH4UI
Unit 10.1 Polarization of Light
Extra Notes and Comments
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