Which of the following types of waves cannot be polarised in ordinary physical experiments?

Introduction / Context:
Polarisation is a property associated with the orientation of oscillations in a transverse wave. Electromagnetic waves such as radio, infrared, and light can show polarisation effects. Sound waves in fluids, including ultrasonic waves, are usually longitudinal, with oscillations in the direction of propagation. This question tests whether students can distinguish between waves that can be polarised and those that cannot in typical laboratory conditions.

Given Data / Assumptions:

• Polarisation is defined as the restriction of the vibration direction of a transverse wave to a particular plane.
• Electromagnetic waves such as radio waves, ultra violet, infrared, and visible light are transverse in nature.
• Ultrasonic waves are high frequency sound waves that travel through media like air, water, or solids.
• In fluids, sound waves are predominantly longitudinal, with compressions and rarefactions along the direction of travel.
• We are considering ordinary laboratory polarisation of waves, not specialised solid state effects.

Concept / Approach:
Polarisation requires that the wave displacement be perpendicular to the direction of propagation so that it can be confined to a particular plane. Transverse waves satisfy this condition and can be polarised. Electromagnetic waves are inherently transverse and can show linear, circular, and elliptical polarisation. In contrast, longitudinal waves have oscillations along the direction of propagation, so there is no transverse direction to select or restrict. Therefore, longitudinal sound waves such as ultrasonic waves in air or water cannot be polarised in the normal sense.

Step-by-Step Solution:
Step 1: Recall that radio, ultra violet, infrared, and visible light are all electromagnetic waves and are transverse.Step 2: Recognise that ultrasonic waves are simply sound waves of very high frequency, typically above the human hearing range.Step 3: Understand that normal sound waves in fluids are longitudinal, with the medium vibrating along the direction of travel.Step 4: Conclude that longitudinal waves like ultrasonic sound cannot be polarised, while transverse electromagnetic waves can be.

Verification / Alternative check:
Experiments with polarising filters easily demonstrate the polarisation of light, radio, and microwave radiation. Polarised sunglasses, for example, reduce glare by blocking certain vibration directions of light. Radio antennas are also oriented to match the polarisation of the transmitted wave. In contrast, no simple polariser exists for ordinary longitudinal sound waves in air, because there is no transverse vibration direction to eliminate. This practical difference supports the theoretical conclusion that ultrasonic sound cannot be polarised in the usual way.

Why Other Options Are Wrong:
Option a is wrong because radio waves can be polarised and often are in communication systems. Option b is incorrect since ultra violet light is electromagnetic and can be polarised. Option c is also incorrect because infrared radiation, being electromagnetic, can be polarised. Option e is wrong as visible light, such as that from a laser, is a classic example of a wave that can be polarised and often has well defined polarisation.

Common Pitfalls:
Students sometimes assume that any wave can be polarised simply because they hear about polarised sound in solids, where certain modes can have transverse components. However, the question refers to the usual longitudinal sound waves in fluids where this does not apply. Another confusion is to think that high frequency alone, as in ultrasonic, might enable polarisation, but frequency does not change the longitudinal or transverse nature. Always remember that polarisation is a feature of transverse waves, and ordinary sound in air or water is longitudinal.

Final Answer:
Ultrasonic waves that are longitudinal sound waves in a medium.