Which physical phenomenon clearly established that light has a transverse nature rather than a purely longitudinal one?

Introduction / Context:
Classical wave theory distinguishes between transverse waves, where oscillations are perpendicular to the direction of propagation, and longitudinal waves, where oscillations are parallel to the direction of propagation. Light, as an electromagnetic wave, is transverse. The phenomenon that most clearly demonstrates this transverse nature is polarisation, which directly involves the orientation of oscillations. This question asks you to recall which phenomenon provided strong evidence that light is transverse.

Given Data / Assumptions:

• We are considering several well known wave phenomena: reflection, interference, diffraction, and polarisation.
• Polarisation depends on the orientation of the electric field oscillations in the light wave.
• Experiments with polarising crystals and filters show changes in intensity depending on orientation.
• Reflection, interference, and diffraction can occur for both transverse and longitudinal waves.
• The question is about which phenomenon uniquely needs transverse oscillations.

Concept / Approach:
Polarisation is defined as the restriction of the vibration direction of a transverse wave to a particular plane. Only transverse waves, where vibrations are perpendicular to the direction of propagation, can be polarised in this way. Longitudinal waves like ordinary sound in air cannot show such polarisation. Reflection, interference, and diffraction can occur for both transverse and longitudinal waves, so they do not by themselves prove that light is transverse. Observations of polarisation of light, especially by crystals like calcite and by polarising filters, were key historical evidence that the vibrations in light are transverse to its direction of travel.

Step-by-Step Solution:
Step 1: Recall the definition of a transverse wave and understand that polarisation is a property of such waves.Step 2: Recognise that light can be polarised by reflection, by transmission through certain crystals, or by passing through sheet polarisers.Step 3: Note that when a polarised beam is passed through a second polariser rotated relative to the first, the transmitted intensity varies, demonstrating directional vibration.Step 4: Conclude that this behaviour has no analogue in simple longitudinal waves and therefore polarisation clearly shows the transverse nature of light.

Verification / Alternative check:
Sound waves in air exhibit reflection, interference, and diffraction, even though they are longitudinal. This confirms that these three phenomena do not require transverse behaviour and cannot be used alone to infer the nature of oscillations. However, no ordinary longitudinal sound wave can be polarised in the same manner as light. Experiments with polarising filters showing complete extinction when their transmission axes are at right angles strongly indicate that light vibrations are confined to a plane, consistent with a transverse electromagnetic wave model.

Why Other Options Are Wrong:
Option a is wrong because reflection occurs for many types of waves, including longitudinal ones, and does not require transverse vibrations. Option b is incorrect since interference patterns also appear in sound and water waves, which do not all have purely transverse oscillations. Option c is not correct because diffraction is common to both transverse and longitudinal waves and does not reveal the orientation of oscillations. Option e refers to refraction, which involves speed and direction changes but again can occur with various wave types and does not by itself prove that light is transverse.

Common Pitfalls:
Students sometimes interpret any striking wave phenomenon as special evidence for the nature of light and may pick interference or diffraction because they are visually impressive. It is important to remember that those phenomena are more general and apply to many kinds of waves. Polarisation, in contrast, is tightly linked to transverse oscillations and is therefore the crucial phenomenon for this question. Keeping this association clear helps in understanding both classical wave optics and modern electromagnetic theory.

Final Answer:
Polarisation of light when it passes through certain crystals or polarising filters.