Polarisation
Polarisation
- Polarisation means making a wave vibrate in a single plane.
- Only transverse waves can be polarised. This is a common exam idea: it is the test that separates the two wave types.
What polarising means
Definition
1 markWhat is meant by plane-polarised wave?
Model answer: A transverse wave whose oscillations are restricted to one direction or plane.
- In unpolarised light, the transverse electric-field direction changes randomly with time. There is no single fixed plane of polarisation.
- A polarising filter only lets through the part of the vibration lined up with its transmission axis. What comes out vibrates in one plane: it is plane-polarised.
vertical vibrationslot selects verticalUnpolarised light has no fixed transverse direction. Plane-polarised light oscillates in one fixed plane. A filter selects that plane.
Why sound cannot be polarised
- Sound is longitudinal: the air vibrates back and forth along the direction of travel.
- There is only that one direction of vibration, so there is no plane to pick out. A longitudinal wave cannot be polarised.
- The same works for microwaves: a metal grid only passes microwaves whose vibration is lined up with the gaps, proving they are transverse.
Your turn9702/23/M/J/25 Q5(a)
State why sound waves cannot be polarised. (9702/23/M/J/25 Q5(a))
Show worked answer
Sound in air is longitudinal. Its particles vibrate parallel to the direction of travel, so there is no transverse vibration plane to select.