Polarisation as Evidence for Transverse Waves
Linear polarisation restricts transverse oscillations to one direction. Ideal crossed polarising filters block light. Longitudinal sound in air cannot be polarised; optical filters are not a test for sound.
What a learner can do afterwards
- Predicts the brightness through two filters as one is turned, including the fully blocked position
- Names polarisation behind glare reduction in sunglasses and behind aerial alignment
- Explains why longitudinal sound in air cannot be polarised and why an unchanged reading with one optical filter does not rule out transverse light
1 · Read
A transverse wave oscillates perpendicular to its travel direction. Linear polarisation restricts those oscillations to one direction. Longitudinal waves have no sideways oscillation direction to select. Observing linear polarisation therefore shows that a wave has transverse oscillations.
A polarising filter has a transmission axis. It passes the electric-field component parallel to that axis. Here, assume ideal filters and no material between them. With fixed unpolarised input, two parallel axes give the greatest output. Turning one axis toward 90 degrees makes the output dimmer. At 90 degrees, the axes are crossed and the second filter transmits no light.
Glare reflected from a horizontal water surface is often partly linearly polarised, with more horizontal electric-field oscillation. Polarising sunglasses usually have a vertical transmission axis. They reduce the horizontal component and cut glare. Partly polarised glare need not disappear completely.
Ordinary sound in air is longitudinal: particles oscillate along its travel direction. It has no transverse vibration direction to select, so it cannot be polarised. Optical filters select light's electric field; they do not test sound's wave type.
A straight receiving rod aerial responds to the radio wave's electric-field component along it. With travel direction and other conditions fixed, a linearly polarised signal gives the strongest reception when rod and field are parallel. This differs from pointing a directional aerial toward a transmitter.
For fixed unpolarised input, one ideal filter transmits half the incident average intensity at every axis angle. A constant reading does not show that light is longitudinal. Keep this first filter fixed, then rotate a second filter to test transmission against the angle between their axes.
One means the incident intensity. Each bar shows half of it passing through one ideal filter. These bars do not show electric-field directions.
Unpolarised light has no preferred transverse electric-field direction over the measurement time. One ideal filter produces light linearly polarised along its transmission axis and transmits half the incident intensity. With linearly polarised light, the electric field oscillates along a fixed line perpendicular to the direction of travel.
Linear polarisation is evidence of transverse oscillations. Ideal crossed filters block light. Sound in air is longitudinal; optical filters do not test it.
Original lesson, questions and flashcards by the dataset authors.
The rewritten readings were written for this dataset. Videos credited to Khan Academy.
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