Edexcel A Level (IAL) Physics-2.17 Plane Polarisation- Study Notes- New Syllabus

Plane Polarisation

Polarisation is a wave property that describes the orientation of the oscillations of a transverse wave. Only transverse waves (e.g., light, radio waves, microwaves, water waves, seismic S-waves) can be polarised. Longitudinal waves (e.g., sound) cannot be polarised because their oscillations occur along the direction of travel.

What Is Plane Polarisation?

Plane polarisation occurs when the oscillations of a transverse wave are restricted to one plane only.

• Unpolarised waves vibrate in many random planes perpendicular to the direction of propagation.
• A plane-polarised wave vibrates in only one plane.
• The direction of propagation remains the same; only the oscillation direction changes.

Example: Light from the Sun or a bulb is unpolarised (vibrates in all planes). Passing it through a polarising filter produces plane-polarised light.

 How Polarisation Occurs

There are several methods:

Characteristics of Plane-Polarised Waves

• Oscillations occur in only a single plane.
• If a polarised wave passes through another polariser:
  • If axes are aligned → wave passes through.
  • If axes are perpendicular → no wave passes (intensity = 0).
• Polarisation changes wave intensity:
  

  \( I_{\text{transmitted}} = I_{\text{incident}} \cos^2\theta \)

(Malus’ law — not required here, but useful conceptually.)

Uses and Applications

• Polarising sunglasses
  • Reduce glare from horizontal surfaces by blocking horizontally polarised light.
• Photography
  • Polarising filters reduce reflections and improve clarity.
• Liquid crystal displays (LCD screens)
  • Use controlled polarisation of light through liquid crystals.
• Stress analysis in materials (photoelasticity)
  • Polarised light shows stress patterns in transparent materials.
• 3D glasses in cinemas
  • Each lens passes light with different polarisation → separate left and right eye images.

 Identifying Polarisation in Experiments

• Rotate a polarising filter in front of a light source:
  
  • Unpolarised light → intensity varies continuously as filter rotates.
  • Plane-polarised light → intensity falls to zero at 90° rotation.
• Using microwaves: A metal grille acts as a polariser by blocking oscillations parallel to its wires.