Multiple choiceQuestion 1 · Recall
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By the end you can explain what polarisation is, describe the difference between unpolarised and plane-polarised waves, and explain why only transverse waves can be polarised.
Light is a transverse electromagnetic wave: its electric and magnetic field oscillations are perpendicular to the direction the wave travels.
But "perpendicular" leaves a whole plane of directions. Imagine looking along the beam: the electric field could oscillate up and down, left and right, or at any angle in between.
In unpolarised light, the oscillations are spread across all of those directions, and the direction changes randomly from moment to moment.
Polarisation forces the oscillations into one direction. The wave still travels forward, but it now wiggles in a single plane.
Plane-polarised wave
A transverse wave whose oscillations are confined to a single plane containing the direction of propagation. The orientation of that plane is the plane of polarisation.
Imagine shaking a rope so that it wiggles randomly up, down and sideways. That is an unpolarised transverse wave.
Now pass the rope through a narrow vertical gap between two fence boards. Only the up-and-down wiggles get through; the side-to-side wiggles hit the boards and are stopped.
The wave emerging from the gap is plane-polarised: it oscillates in one plane only.
A polarising filter does exactly this to light. It transmits the component of the electric field along its transmission axis and absorbs the perpendicular component.
The analogy breaks in one way: a real filter does not physically block a rope, it absorbs light whose electric field is perpendicular to its axis. But the idea — keep one direction, remove the rest — is the same.
Sound is a longitudinal wave. Its particles oscillate along the direction the wave travels: backwards and forwards, not side to side.
There is no perpendicular plane in which the oscillation could be confined, so there is nothing to polarise. You cannot polarise sound in air, and no "sound Polaroid" exists.
This gives a neat test. If a wave can be polarised, it must be transverse. The fact that light can be polarised was historically important evidence that light is a transverse wave, not a longitudinal one.
Polarisation makes a wave travel in one direction.
The wave already travels in one direction. Polarisation restricts the oscillations, not the direction of travel. The beam still moves forward; it just wiggles in one plane.
Polarised light is brighter than unpolarised light.
A filter removes some of the light, so the transmitted beam is usually less intense. Polarisation changes the direction of oscillation, not the amount of energy in a helpful way.
Sound can be polarised if it passes through a narrow slit.
A slit can diffract sound, spreading it out, but it cannot polarise it. Sound is longitudinal, and polarisation is a property of transverse waves only.
Only light can be polarised.
Light is an electromagnetic wave with an oscillating electric field and an oscillating magnetic field, at right angles to each other and both perpendicular to the direction of travel.
By convention, the plane of polarisation of an electromagnetic wave is the plane containing the electric field oscillation and the direction of propagation. The magnetic field oscillates in the perpendicular plane.
When light reflects from a non-metallic surface such as water or glass, the reflected beam is partly polarised parallel to the surface. That is why polarising sunglasses, with their transmission axis vertical, reduce glare from horizontal surfaces: the reflected glare is mostly horizontally polarised, so the vertical filter absorbs it.
Any transverse wave can be polarised, including microwaves, radio waves and waves on a string. Light is just the most familiar example.