IB DP Physics: SL复习笔记4.3.4 Polarisation

Polarisation

  • Transverse waves can oscillate in any plane perpendicular to the direction of motion (and energy transfer) of the wave
  • Such waves are said to be unpolarised
  • When a transverse wave is polarised, its electric field is only allowed to oscillate in one fixed plane perpendicular to the direction of motion of the wave
    • A transverse wave can be vertically polarised, horizontally polarised, or polarised in any direction in between

7.1.5.1-Polarised-waves-diagram

Diagram showing the displacement of unpolarised and polarised transverse waves

  • Since longitudinal waves oscillate in the same direction as the direction of motion of the wave, polarisation of longitudinal waves cannot occur
  • Methods of polarisation include polarising filters and reflection from a non-metallic plane surface

Polarising Filters

  • Light waves can be polarised by making them pass through a polarising filter called a polariser
  • The filter imposes its plane of polarisation on the incident light wave
  • A polariser with a vertical transmission axis only allows vertical oscillations to be transmitted through the filter (A)
  • If vertically polarised light is incident on a filter with a horizontal transmission axis, no transmission occurs (B), and the wave is blocked completely

7.1.5.1-Waves-through-a-polariser

Diagram showing an unpolarised and polarised wave travelling through polarisers

Polarisation via Reflection

  • When unpolarised light reflects from a smooth non-metallic surface, partial plane polarisation always occurs
  • Reflected light is polarised in a plane parallel to the reflecting surface
    • This means if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane
  • Polarising sunglasses use this property of reflection in order to reduce the glare coming from a reflective surface (e.g. water)

3.1.3-Polaroid-Sunglasses-1

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

3.1.3-Polaroid-Sunglasses-2

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

  • As a result, objects under the surface of the water can be viewed more clearly

Polarised, Reflected & Transmitted Beams

  • Beams can be polarised, reflected or transmitted
  • When beams are polarised, the oscillations of the waves are made to oscillate only in one plane
    • This affects the intensity of the waves
  • Diagrams demonstrating polarisation will include a double-headed arrow showing the plane of polarisation of the wave

4-3-4-diagram-of-polarisation_sl-physics-rn

  • When beams are reflected, they bounce back in the direction that they have come in by the same angle
  • When beams are transmitted, they travel straight through the medium
    • In both these cases, the light can still be polarised
  • Plane polarisation is when the direction of the vibrations stays constant over time, and the vibrations are 100 % restricted in that direction
  • Partial polarisation is when there is some restriction to the direction of the vibrations but not 100 %
  • This can be seen when an unpolarised light beam travels from air to glass
    • The light is initially unpolarised when incident on the glass
    • Some of the beam is reflected, partially polarising it
    • Some of the beam is transmitted and refracted, also partially polarising it

4-3-5-polarised-reflected-and-transmitted-beam_sl-physics-rn

 

 

 

 

 

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