Light Waves and Their Nature

Light, as we understand it in physics, exhibits wave-like properties. These waves are a form of electromagnetic radiation, meaning they consist of oscillating electric and magnetic fields that travel through space. Waves can broadly be classified into two types based on the direction of particle oscillation relative to the direction of wave propagation: longitudinal waves and transverse waves.

• Longitudinal Waves: In these waves, the particles of the medium oscillate parallel to the direction of wave propagation. Sound waves are a common example of longitudinal waves.
• Transverse Waves: In contrast, for transverse waves, the particles of the medium oscillate perpendicular to the direction of wave propagation. Waves on a string or ripples on water are examples of transverse waves.

The question asks which phenomenon establishes the transverse nature of light waves. This means we are looking for an experiment or observation that can only be explained if light waves are transverse, not longitudinal.

Polarisation and Light Waves

Polarisation is the phenomenon that conclusively demonstrates the transverse nature of light waves. When light is unpolarised, its electric field vectors oscillate in all possible directions perpendicular to the direction of wave propagation. A polariser is a device that allows only those light waves to pass through that have their electric field vectors oscillating in a specific plane or direction.

Here's how polarisation establishes the transverse nature:

• Imagine a rope passing through two parallel slits. If you shake the rope vertically, the wave propagates, and it can pass through the vertical slits. If you rotate one slit horizontally, the wave cannot pass through. This demonstrates the transverse nature of the rope wave, where oscillations have a specific direction perpendicular to propagation.
• Similarly, if light waves were longitudinal, their oscillations would be along the direction of propagation. In such a case, there would be no preferred "plane of oscillation" perpendicular to the direction of travel that could be blocked or filtered by a polariser. Regardless of how a polariser is oriented, a longitudinal wave would pass through unaffected, as its oscillations are always along the propagation direction.
• However, since light can be polarised (i.e., its oscillations can be restricted to a single plane by passing through a polariser), it implies that the oscillations must be perpendicular to the direction of wave propagation, just like the rope example. This ability to select a specific oscillation plane proves that light waves are transverse.

Diffraction and Wave Nature

Diffraction is the phenomenon where waves spread out as they pass through an aperture or around obstacles. This effect is common to all types of waves, whether longitudinal (like sound waves) or transverse (like water waves or light waves). While diffraction confirms the wave nature of light, it does not specifically distinguish between longitudinal and transverse waves. Both types of waves can exhibit diffraction patterns.

Photoelectric Effect and Particle Nature

The Photoelectric effect is the phenomenon where electrons are ejected from a material when light shines on it. This effect is best explained by considering light as composed of discrete energy packets called photons (particle nature of light), rather than continuous waves. The energy of the ejected electrons depends on the frequency of light, not its intensity. This phenomenon highlights the particle aspect of light, known as wave-particle duality, but does not provide evidence for its transverse or longitudinal wave nature.

Compton Effect and Particle Nature

The Compton effect describes the scattering of a photon by a charged particle, usually an electron, resulting in a decrease in energy (increase in wavelength) of the photon. This effect provides strong evidence for the particle nature of light (photons) and the conservation of momentum in photon-electron collisions. Like the photoelectric effect, it supports the quantum theory of light and its particle characteristics, but it does not specifically address or establish the transverse nature of light waves.

Conclusion on Light's Transverse Nature

Among the given options, only Polarisation provides direct and conclusive evidence that light waves are transverse. The other phenomena (Diffraction, Photoelectric effect, Compton effect) either demonstrate the general wave nature, or the particle nature of light, but do not specifically establish its transverse characteristic.