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
- An ordinary (unpolarised) transverse wave vibrates in every plane at once, all at right angles to the travel direction.
- 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.
vibrates
up & downvertical slot
= transmission axispasses throughUnpolarised = vibrating in all planes. Plane-polarised = vibrating in one plane. A filter turns the first into the second.
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 turn— tap to reveal the worked answer (9702/23/M/J/25 Q5)
State why sound waves cannot be polarised. (9702/23/M/J/25 Q5)
Answer: sound is a longitudinal wave; its vibration is along the direction of travel, so it has no plane of vibration to restrict.
Your turn— tap to reveal the worked answer (9702/11/M/J/25 Q28)
Which group contains only waves that can be polarised? (9702/11/M/J/25 Q28)
Answer: a group of transverse waves only (e.g. light, radio, water waves). Any group containing sound is wrong.