What is meant by plane polarised light ? Can sound waves be polarised ? Briefly discuss polarisation by scattering.

Explanation

Plane polarised light. The phenomenon of restricting the vibrations of light (electric vector) in a particular plane is called polarisation of light. And the light in which the vibrations of light (electric vector) are restricted to a particular plane is called plane polarised light.
No. Sound waves cannot be polarised because sound waves are longitudinal waves and polarisation is possible only in transverse waves.
Polarisation by scattering
When unpolarised light [travelling along X-axis (say)] is made to pass through a medium containing particles of size comparable to wavelength of light, the light gets scattered. If the observation is made perpendicular to the direction of incident waves (say along Y-axis or Z-axis), the light is found to be plane polarised.

If the observer is looking along Z-axis, then the polarised light has plane of vibrations parallel to Y axis, and if the observer is looking along Y-axis, then the polarised light has plane of vibration parallel to Z-axis as shown in the figure. This is the method of polarising light by scattering.