Polarisation as evidence for the nature of transverse waves.

The idea

Polarisation is the restriction of a transverse wave's oscillations to one plane. In an unpolarised transverse wave, the oscillation direction is randomly distributed around the line of travel, so every plane perpendicular to the propagation direction contains oscillations. A polarising filter transmits only the component of oscillation parallel to its transmission axis, reducing intensity and producing plane-polarised light. Crucially, longitudinal waves cannot be polarised because their oscillations lie along the direction of travel, leaving no perpendicular plane to select. If a wave can be polarised, it must be transverse; this is the evidence.

For example: Rotating a second polariser through 90° relative to the first extinguishes transmitted light, whereas sound cannot be silenced this way.

What earns the marks

• Polarisation restricts oscillations to a single plane containing the direction of propagation.
• Unpolarised transverse waves oscillate in all planes perpendicular to the direction of travel.
• A polarising filter transmits only oscillations parallel to its transmission axis, so intensity falls.
• Longitudinal waves oscillate parallel to the direction of travel and therefore cannot be polarised.
• Observation of polarisation is direct evidence that the wave is transverse, because only transverse waves have perpendicular oscillation directions that can be selected.

Common mistakes and how to put them right

• Saying polarisation removes half the oscillations in every direction.

  Put it right: It selects one plane and reduces intensity according to the angle between axes.

• Claiming sound can be polarised because it is a wave.

  Put it right: Sound is longitudinal, so its oscillations are along the direction of travel and no perpendicular plane exists to select.

• Confusing polarisation with reflection or diffraction.

  Put it right: Polarisation is a property of transverse oscillations, not a change in direction or spreading.

Exam tips

• State explicitly that the oscillations are confined to one plane and link this to the transverse nature of the wave.
• Use the polariser pair test: rotating the analyser changes transmitted intensity, reaching a minimum when axes are perpendicular.
• Contrast with a longitudinal wave such as sound to show why polarisation is impossible for it.