Polarisation is a specific property of transverse waves that describes the geometrical orientation of their oscillations. Transverse waves are polarised by restricting the direction of their oscillations to a single plane or direction.
Understanding Polarization in Transverse Waves
Polarization specifies the orientation of the oscillations in a transverse wave. A key characteristic of transverse waves, as stated in the provided reference, is that the direction of the oscillation is perpendicular to the direction of motion of the wave. This fundamental geometry is what makes polarisation possible for transverse waves, unlike longitudinal waves (where oscillations are parallel to the wave's motion).
Imagine a transverse wave moving forward. Its oscillations happen side-to-side, up-and-down, or anywhere in the plane perpendicular to the direction the wave is travelling.
Why Only Transverse Waves?
Let's briefly contrast transverse and longitudinal waves:
- Transverse Waves: Oscillations are perpendicular to wave motion (like waves on a string or light waves).
- Longitudinal Waves: Oscillations are parallel to wave motion (like sound waves).
Because longitudinal wave oscillations are always back and forth along the direction of travel, there's no "orientation" in a perpendicular plane to specify or restrict. Transverse waves, however, have oscillations in a plane perpendicular to propagation, allowing for different orientations within that plane (vertical, horizontal, diagonal, circular, elliptical). Polarisation is the process or state of having a preferred orientation within this perpendicular plane.
How Polarisation Happens
Most natural sources of transverse waves (like light from a conventional bulb) produce waves with oscillations occurring randomly in all possible directions within the plane perpendicular to the direction of travel. These waves are considered unpolarised.
To polarise a transverse wave means to select or favour oscillations in a specific direction or plane. This is typically achieved by passing the wave through a polarising filter or interacting it with a surface or material that absorbs or reflects waves oscillating in other directions.
| Wave State | Oscillation Direction in Perpendicular Plane | Polarisation |
|---|---|---|
| Unpolarised | Randomly oriented in all directions | Not polarised |
| Polarised | Restricted to a single direction or plane | Linear, Circular, or Elliptical Polarised |
Practical Examples: Light Waves
Light is a classic example of a transverse wave (specifically, an electromagnetic wave) that can be polarised.
- Unpolarised Light: Light from the sun or a standard lamp vibrates in all possible directions perpendicular to its path.
- Linearly Polarised Light: When unpolarised light passes through a linear polarising filter (like the lens of polarised sunglasses), only the light waves oscillating in a specific direction (aligned with the filter's transmission axis) are allowed through. The filter absorbs or reflects the light oscillating in other directions. This results in light waves whose oscillations are confined to a single line or plane – hence, linearly polarised.
- Other Types: Light can also be circularly or elliptically polarised, where the oscillation direction rotates or traces an elliptical path in the perpendicular plane over time.
In summary, transverse waves are polarised by taking waves oscillating in multiple directions perpendicular to their path and restricting those oscillations to a specific, preferred direction or plane. This is only possible because of the fundamental geometry of transverse waves, where the oscillations occur perpendicular to the wave's motion.
