Which one of the following cannot be polarised?

The correct answer is Option (2) → Sound waves

Polarisation means restricting the vibrations of a wave to one direction perpendicular to its direction of travel. Only transverse waves can be polarised; in a longitudinal wave the vibration is along the direction of travel, so there is no perpendicular direction to select.

Option (1) \( \gamma \)-rays — Can be polarised. They are electromagnetic waves, which are transverse.

Option (2) Sound waves — Cannot be polarised. Sound in air is a longitudinal wave: the air particles vibrate along the direction of travel.

Option (3) Microwaves — Can be polarised. Microwaves are electromagnetic waves, so they are transverse.

Option (4) Ultraviolet rays — Can be polarised. Ultraviolet rays are also transverse electromagnetic waves.

Note: A similar question was also asked in CUET (UG) 2023 on 23-05-2023 (Slot 3).

NCERT (2026-27):
Points to Ponder – “… Most interference and diffraction effects exist even for longitudinal waves like sound in air. But polarisation phenomena are special to transverse waves like light waves.”