AQA A Level Physics复习笔记3.1.3 Polarisation

Polarisation

  • Polarisation is when:

Particle oscillations occur in only one of the directions perpendicular to the direction of wave propagation

 

  • Polarisation can only occur in transverse waves
    • This is because transverse waves oscillate in any plane perpendicular to the propagation direction

     

  • When transverse waves are polarised, this means:
    • Vibrations are restricted to one direction
    • These vibrations are still perpendicular to the direction of propagation / energy transfer

     

  • The difference between unpolarised and polarised waves is shown in the diagram below:

7.1.5.1-Polarised-waves-diagram

Diagram showing the displacement of unpolarised and polarised transverse waves

 

  • Longitudinal waves (e.g. sound waves) cannot be polarised
    • This is because they oscillate parallel to the direction of travel

     

  • Waves can be polarised through a polariser or polarising filter
    • This only allows oscillations in a certain plane to be transmitted

     

 

7.1.5.1-Waves-through-a-polariser

Diagram showing an unpolarised and polarised wave travelling through polarisers

 

  • Diagram A shows:
    • Only unpolarised waves can be polarised

     

  • Diagram B shows:
    • When a polarised wave passes through a filter with a transmission axis perpendicular to the wave, none of the wave will pass through

     

  • Light can also be polarised through reflection, refraction and scattering

Investigating Light Intensity with Two Polarisers

  • If an unpolarised light source is placed in front of two identical polarising filters, A and B, with their transmission axes parallel:
    • Filter A will polarise the light in a certain axis
    • All of the polarised light will pass through filter B unaffected
    • In this case, the maximum intensity of light is transmitted

     

3.1.3-Polarised-Light-Intensity

When both polarisers have the same transmission axis, the intensity of the transmitted light is at its maximum

 

  • As the polarising filter B is rotated anticlockwise, the intensity of the light observed changes periodically depending on the angle B is rotated through
  • When A and B have their transmission axes perpendicular to each other:
    • Filter A will polarise the light in a certain axis
    • This time none of the polarised light will pass through filter B
    • In this case, the minimum intensity of light is transmitted

     

 

3.1.3-Polarised-Light-Intensity-2

When one of the polarisers is rotated through 90°, the intensity of the transmitted light drops to zero

 

  • The resulting graph of the light intensity with angle, as the second polariser is rotated through 360°, looks as follows:

3.1.3-Intensity-v-Angle-Graph

Graph showing how the intensity of the transmitted beam varies with the angle between the transmission axes of the two polarisers

 

  • In real life, the intensity of the unpolarised electromagnetic wave reduces after it passes through a polarising filter

Worked Example

Which statement below describes a situation in which polarisation should happen?A.      Radio waves pass through a metal gridB.      Surface water waves are diffractedC.      Sound waves are reflectedD.      Ultrasound waves pass through a metal grid

      ANSWER: A

  • Radio waves are transverse waves - they can be polarised by a metal grid so only the waves that fit through the grid will be transmitted, therefore, A is correct
  • B cannot be correct as waves are not polarised when diffracted, but are polarised only when reflectedrefracted or scattered
  • C & D cannot be correct as polarisation only occurs for transverse waves, therefore, C & D can be ruled out as sound and ultrasound are both longitudinal waves

Exam Tip

You may be expected to describe the intensity, or even draw the graph of intensity v angle, for light with two polarisers, however, the good news is that you won't be expected to perform any calculations (Malus's Law) in relation to this.

Applications of Polarisers

Polaroid Sunglasses

  • Polaroid sunglasses are glasses containing lens with polarising filters with transmission axes that are vertically oriented
    • This means the glasses do not allow any horizontally polarised light to pass through

     

3.1.3-Polaroid-Sunglasses-1

Polaroid sunglasses contain vertically oriented polarising filters which block out any horizontally polarised light

 

  • When light is reflected from a reflective surface e.g. the surface of water or a wet road, it undergoes partial plane polarisation
    • This means if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane

     

  • Therefore, polaroid sunglasses are useful in reducing the glare on the surface of the water (or any reflective surface) as the partially-polarised light will be eliminated by the polarising filter
  • As a result of this, objects under the surface of the water can be viewed more clearly

 

3.1.3-Polaroid-Sunglasses-2

When sunlight reflects off a horizontal reflective surface, such as water, the light becomes horizontally polarised. This is where polaroid sunglasses come in useful with their vertically aligned filter

 

Polaroid Photography

  • Polaroid cameras work in the same way as polaroid sunglasses
    • They are very useful for capturing intensified colour and reducing glare on particularly bright sunny days

     

  • Polarising filters also enable photographers to take photos of objects underwater
    • This is because the light reflected on the surface of the water is partially polarised in the horizontal plane
    • This glare is eliminated by the polarising lens
    • However, the light from the underwater object is refracted by the surface of the water, not reflected, so it is not plane-polarised
    • Therefore, the light from the underwater object is more intense than the glare and shows up much more brightly in the photo

     

3.1.3-Polaroid-Photography

As well as giving a cool look to photographs, polaroid filters are extremely useful for reducing glare in photos and snapping pictures of objects underwater

 

Polarisation of Radio & Microwave Signals

  • Radio and television services are broadcast either horizontally-polarised or vertically-polarised
  • Therefore, the reception aerial needs to be mounted flat (horizontal), or on its side (vertical),
    • The particular orientation of an aerial will depend on the transmitter it is pointing towards and the polarity of the services being broadcast

     

3.1.3-Aerial-Polarisation

Broadcasting towers always transmit either vertically or horizontally polarised signals. This is why aerials must be positioned accordingly otherwise they won't pick up the TV signal correctly

 

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