Physics › Waves › Longitudinal, transverse and polarisation
Longitudinal, transverse and polarisation
Waves come in two kinds, set apart by the direction their particles oscillate. One experiment, polarisation, tells the kinds apart, and in doing so it settled what light is.
Builds on Progressive waves.
IN THIS TOPIC
- Classify a wave as transverse or longitudinal from the direction of its oscillations.
- Describe what a polarising filter does to unpolarised light, and what a second, crossed filter does next.
- Explain why polarisation is evidence that light is transverse, and why sound can never be polarised.
WHAT YOU PROBABLY THINK
Any wave can be polarised if you build the right filter.
Two ways to oscillate
In a transverse wave, the oscillations are at right angles to the direction the energy travels. Waves on a rope are transverse, and so are all electromagnetic waves, from radio to gamma rays, which is why every one of them travels at the same speed c in a vacuum.
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. The particles bunch together and spread apart as the wave passes, forming compressions, where the pressure is highest, and rarefactions, where it is lowest. Sound is the important example: air particles shuffle back and forth along the same line the sound travels.
Polarisation
An ordinary lamp sends out light whose oscillations point in every direction perpendicular to the ray, changing randomly and rapidly. This is unpolarised light. A polarising filter transmits only the component of the oscillation along one direction, its transmission axis. What comes out is plane polarised: the oscillations now lie in a single plane containing the ray.
Hold up a second filter and rotate it. The transmitted intensity falls as the angle between the two transmission axes grows, and at 90°, with the filters crossed, the light is blocked completely. The first filter left only vertical oscillations, and a horizontal slot passes none of a vertical oscillation.
Here is the argument that matters. Filtering by oscillation direction only means anything if the oscillations are perpendicular to the ray in the first place. A longitudinal wave oscillates along its direction of travel, the one direction no filter orientation can distinguish. So the fact that light can be polarised is direct evidence that light is a transverse wave, and sound can never be polarised at all.
Where you meet it
Sunlight reflected from water or wet road is partially polarised horizontally. Polarising sunglasses mount their filters with a vertical transmission axis, so they remove that glare while passing most other light.
Television and radio signals are transmitted plane polarised. An aerial receives best when its rods are lined up with the plane of polarisation of the incoming wave, which is why the rooftops of one town all point their aerials the same way, matched to the local transmitter.
THE EXAM BIT
- Definitions earn their marks from the comparison: oscillations perpendicular to (transverse) or parallel to (longitudinal) the direction of energy transfer. Name the two directions being compared.
- “Why can sound not be polarised?” wants two steps: sound is longitudinal, and polarisation is only possible for transverse waves. Stating only one of them is a half answer.
- Rotating one filter above another: intensity is greatest with the axes parallel and zero with them at 90°. Describe the variation; no equation for it is required.
- Polarisation questions about aerials are asking for alignment: the aerial must be parallel to the plane of polarisation of the signal.
- Say plane polarised, and name the plane where you can. “The light is filtered” describes the apparatus, and scores nothing.
CHECK YOURSELF
Ultrasound is used to image a fetus, and light is used to read a barcode. One of these waves could in principle be polarised. Which one, and why?
Show a hint
Classify each wave first. Which way does each one oscillate compared with its direction of travel?
Show the answer
The light. Light is an electromagnetic wave, so it is transverse, and its oscillations are perpendicular to the ray. A filter can select one oscillation direction, which is what polarisation is.
Ultrasound is sound: a longitudinal wave. Its particles oscillate along the direction of travel, so there are no perpendicular oscillation directions to select between, and no filter orientation can polarise it.
Only transverse waves polarise.
Polarised light is the proof light is transverse.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.