1© Manhattan Press (H.K.) Ltd. 9.4 Polarization of light waves Polarization by selective absorption...

© Manhattan Press (H.K.) Ltd. 1

9.4 Polarization 9.4 Polarization

of light wavesof light waves

• Polarization by selective Polarization by selective absorptionabsorption

• • Polarization by reflectionPolarization by reflection

• • Polarization by scatteringPolarization by scattering

• • Uses of polarization of lightUses of polarization of light


PolarizationPolarization

9.4 Polarization of light waves (SB p. 39)

It only occurs in transverse waves, such as electromagnetic waves.

Polarization is the restriction of the vibrations in

a wave so that the vibrations occur in a single

plane.


1. Polarized light & non-polarized light1. Polarized light & non-polarized light


- A light wave is emitted when an electron orbiting round a nucleus drops from a higher energy level to a lower energy level.

- The transition of the electron produces a light wave.


Polarized lightPolarized light


Vertical polarized light wave from an electron orbiting in the vertical plane.

Horizontal polarized light wave from an electron orbiting in the horizontal plane.


Non-polarized lightNon-polarized light


- The light from a source such as candle flame, a filament lamp or the sun is non-polarized.

- This is because the light waves are from different atoms whose electrons experiencing the transition are orbiting in different planes in all directions.


PolarizerPolarizer


A polarizer allows electric field of the light to vibrate in only one direction and simultaneously absorbs all other electric fieldsvibrating in different directions, resulting in the polarized light.

The direction of vibration of the polarized light is known as the polarization axis.


PolarizerPolarizer


If Io is the intensity of light incident on a polarizer,

the intensity of light transmitted by the polarizer is


Viewing with a polarizerViewing with a polarizer



AnalyserAnalyser


Since the intensity of light (I) is directly proportional to the square of the amplitude of vibration, I (A cosθ)2, the intensity of light emerging from analyser Q is:

I = Io cos2θ [Io A2 ]


Variation of intensityVariation of intensity


If analyser Q is now rotated slowly about the direction of propagation, the intensity of the transmitted light decreases as the angle θ increases from 0°to 90°.

When θ = 90°, no light passes through it. The variation of intensity with the angle θis shown below:



- If θ is increased further, the intensity increases until a maximum is reached when θ = 180°.

Variation of intensityVariation of intensity

- For θ from 180° to 270°, the intensity again decreases and reaches zero when θ = 270°.

- For θ from 270° to 360°, the intensity of the transmitted light increases to a maximum until θ = 360°.


Testing of polarized lightTesting of polarized light


A piece of polarizer held between the eyes and a pair of glasses. It can be used to determine whether a beam of light is polarized.



The polarizer is then slowly rotated about the axis of vision. If the intensity of lightremains unchanged, then the incident beam is not polarized.

If the intensity of the transmitted light varies as the polarizer is rotated and at certain angles of rotation, no light is transmitted, then the light is plane polarized.

Testing of polarized lightTesting of polarized light

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Example 3Example 3

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Example 4Example 4


Polarization by selective absorptionPolarization by selective absorption


If a polarizer is used to select one specific direction of vibration of the wave, then the process is called polarization by selective absorption.


PolaroidPolaroid


- The modern form of the polaroid is made from a plastic sheet which is highly strained to align the molecules in one direction.

- The molecules have long chains which allow the electric field to set the electrons to move freely along the chain.

- Therefore, the energy of electromagnetic waves is transformed into internal energy.

- The electric field in this direction is thus absorbed.


Polarization by reflectionPolarization by reflection


Brewster’s Law

Rotate the polaroid until reaching the minimum intensity and then keep its orientation fixed. Change the viewing angle until the intensity drops to almost zero.

This angle of incidence (i) at the reflecting surface satisfies the following equation:

tan i = n

where n is the refractive index of the material.

This is known as Brewster’s Law and i is Brewster’s angle (57º)


Brewster’s LawBrewster’s Law


From Snell’s Law,

refractive index,

where r is the angle of refraction.

From Brewster’s Law,

tani = n

when polarization is complete.



Brewster’s LawBrewster’s Law

Hence, sinr = cosi

= sin(90 i )

r = 90 i

i + r = 90

That is,

The reflected light is completely polarized when the reflected ray and the refracted ray are at right angles.



Explanation of polarization by reflectionExplanation of polarization by reflection



Polarization of scatteringPolarization of scattering

- If light propagates through a gas (or liquid), the electrons in the gas can absorb and re-radiate part of the light.

- The absorption and re-radiation of light by the gas is called scattering.

- This explains how the sunlight reaching above an observer on the earth is partially polarized as shown.



Study polarization of scattering in lab.Study polarization of scattering in lab.

1. Add a few drops of milk to make a transparent tank of water cloudy.

2. Emit a laser beam (such as He-Ne laser, red light) into the tank.

3. View the scattered light at right angle and other different positions by using a polaroid.

4. Rotate the polaroid and observe the light carefully.



The resultsThe results

The results show that the scattered light is plane polarized at positions A and B, i.e. at right angles to the incident ray.

Obviously, the light passing straight through the tank at position C remains unpolarized.

The light scattered at other positions such as position D are partially polarized.



Uses of polarization of lightUses of polarization of light

Sunglasses use polaroid to reduce the glare from the reflected sunlight.

This is because reflected light from a shiny horizontal surface (e.g. the sea) is partially horizontally polarized.

The polaroid used has its polarization axis in vertical.

Then most of the reflected light will be absorbed by the sunglasses and the intensity of the light entering the eyes is very much reduced.



Receiving radio wave signalsReceiving radio wave signals

Radio signals used for transmitting information are usually polarized. Three common radio waves in HK broadcasting as follows:

AM radio

FM radio

TV



AM radioAM radio

AM bands radio waves are usually vertically polarized.

The radio waves set the electrons to vibrate up and down along in the vertical antenna.

This creates a high frequency alternating current into the circuit inside the radio.



FM radioFM radio

FM radio broadcasts in the VHF band (around 100 MHz) and is horizontally polarized.

Thus, FM antennas should be horizontal.



TV aerialTV aerial

TV broadcasts in the UHF band (around 400 MHz).

In Hong Kong, the TV signals make use of both vertical and horizontal polarization in order to save bandwidth.

Therefore, the direction of the TV antennas is different in different districts.

Although there is a set of receivers on an antenna, only the last one is connected to the television cable electrically.

Others are actually reflectors that focus the signals on the last receiver.



NoteNote

The receiver and reflectors are parallel to the direction of the electric field of the TV signals while the supporter of the antenna is along the direction of propagation of the waves.



Determining the concentration of Determining the concentration of a solutiona solution

When polarized light passes through a certain solution, the plane of polarization is rotated.

The angle of rotation increases as the concentration of the solution increases.

This property of rotating the plane of polarization is known as optical activity.



Determining concentration of a sugar Determining concentration of a sugar solution – by polarimetersolution – by polarimeter

- By viewing the transmitted light through a tube containing sugar solution using a second polaroid which is rotated, the angle through which the plane of polarization has turned can be measured.

- Hence, the concentration of the sugar solution can be determined. This device is called a polarimeter.



Analysing stressAnalysing stress

If a piece of glass which is strained is placed between two pieces of polaroid with their polarization axes crossed at right angles, a stress pattern which consists of lines of various colours is observed.

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More to Know 2More to Know 2

This is because light of different wavelengths are rotated by different amounts.

This effect can be used to analyse some plastic structures like perspex and polythene.



3-D movie3-D movie

In three-dimensional movies, a pair of pictures are taken a short distance apart.

The pictures are then projected onto a screen through a pair of projectors fitted with polarized filters.

The polarization axes of the filters are at right angles to each other – one horizontal and the other vertical.

To the naked eye, the pictures on the screen look blurred.



3-D movie3-D movie

- To see the pictures in 3-D, the viewer needs to wear glasses made of polaroid as shown. Each eye sees a separate picture, just as in real life.

- The brain then interprets the two pictures as a single picture with depth.


End



QQ: : A beam of unpolarized light passes through polarizers P1, P2

and P3 as shown in the figure.

The polarization axis of polarizers P1 and P3 are vertical but

that of polarizer P2 is inclined at 30° to the vertical. To

obtain the minimum intensity for the emergent beam,

through what angle must polarizer P3 be rotated in the

clockwise direction? Solution


Solution:Solution:

The plane of polarization of the beam that passes through P2 is inclined at an angle of 30° to the vertical.

For the intensity of the emergent beam from P3 to be a minimum, the polarization axis of P3 must be perpendicular to the plane of polarization of the light incident on it.

Hence, P3 must be rotated through (90° + 30°) = 120°.

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QQ: :

Two pieces of polarizers are arranged such that their polarization axes are parallel and vertical, as shown in the

figure. The intensity of the emergent light is I0. Through

what angle must polarizer Q be rotated so that the intensityof the emergent light decreases to ?

Solution021I



Solution:Solution:

When the polarization axis of polarizer Q is at an angle θ to the vertical, the intensity of the emergent light is I = I 0 cos2θ

When

cos2θ=

cosθ=

θ= 45

200 cos 2

II

I

21

21

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Stress pattern on windscreen of cars

The curved windscreen of a car is always under strain. Reflected light which is partially horizontally polarized after falling on the windscreen has its plane of polarization rotated. A driver wearing polaroid sunglasses with a vertical polarization axis will be able to observe the stress pattern which is in the form of parallel bands identical to interference fringes.

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1© Manhattan Press (H.K.) Ltd. 9.4 Polarization of light waves Polarization by selective absorption...

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