The transverse nature of electromagnetic waves is most clearly demonstrated by which phenomenon of light?

Introduction / Context:
Electromagnetic waves, such as light, radio waves, and X rays, are known to be transverse waves. In a transverse wave, the oscillations of the electric and magnetic fields are perpendicular to the direction of propagation. Several optical phenomena demonstrate wave behaviour, but one particular effect provides strong evidence that these oscillations are transverse rather than longitudinal. This question asks which phenomenon clearly reveals the transverse nature of electromagnetic waves.

Given Data / Assumptions:

• We are considering basic wave phenomena: reflection, polarisation, interference, and diffraction.
• Electromagnetic waves travel through vacuum and media with electric and magnetic field components perpendicular to the direction of propagation.
• The question focuses on distinguishing transverse and longitudinal waves.
• We assume school level understanding of these phenomena.

Concept / Approach:
Polarisation is the phenomenon in which the vibrations of a transverse wave are restricted to a single direction or plane. Only transverse waves can be polarised because they have a set of possible vibration directions perpendicular to propagation. Longitudinal waves, like sound in air, have oscillations only along the direction of propagation, so there is no meaningful way to restrict vibrations to one direction. Reflection, interference, and diffraction occur for both transverse and longitudinal waves and do not by themselves prove the transverse nature. Therefore, the observation and control of polarisation in light strongly supports the conclusion that electromagnetic waves are transverse.

Step-by-Step Solution:
Step 1: Recall the definition of a transverse wave: the oscillations are perpendicular to the direction of travel. Step 2: Understand that polarisation involves selecting waves with vibrations only in one direction, for example, vertical or horizontal, within the plane perpendicular to propagation. Step 3: Recognise that only a wave with multiple possible vibration directions perpendicular to propagation can be filtered this way, which is characteristic of transverse waves. Step 4: Note that experiments with polarising filters show that two polarising sheets crossed at 90 degrees can block almost all light, demonstrating that the oscillations have directional orientation. Step 5: Conclude that because light can be polarised, it must be a transverse wave, and therefore polarisation is the phenomenon that evidences its transverse nature.

Verification / Alternative check:
Consider sound waves in air, which are longitudinal. They show reflection, interference, and diffraction, but they cannot be polarised in the same sense as light. All displacements are along the line of travel, so there is no transverse direction to select. In contrast, electromagnetic waves can be linearly, circularly, or elliptically polarised, and many optical devices such as sunglasses and liquid crystal displays rely on this property. This difference between light and sound confirms that polarisation is the unique phenomenon that reveals the transverse nature of electromagnetic waves.

Why Other Options Are Wrong:
Reflection of light from a mirror: Reflection is a general wave phenomenon that does not distinguish between transverse and longitudinal waves. Interference of coherent light beams: Interference patterns arise whenever coherent waves superpose; both sound and water waves exhibit interference, so it does not by itself prove transversality. Diffraction of light at narrow openings: Diffraction occurs for many types of waves and also does not uniquely indicate whether a wave is transverse or longitudinal.

Common Pitfalls:
Learners sometimes think that any wave phenomenon involving light automatically reveals its full nature. However, only polarisation directly requires oscillations to have a direction perpendicular to propagation. It is important to separate the idea of wave behaviour in general from the specific question of whether the wave is transverse or longitudinal. Remember that interference and diffraction tell you that light is a wave, whereas polarisation tells you what kind of wave it is.

Final Answer:
The transverse nature of electromagnetic waves is most clearly demonstrated by the polarisation of light waves.