Which of the following phenomena most clearly demonstrates the particle nature of light?

Introduction / Context:
Light exhibits both wave-like and particle-like behaviour, a concept known as wave particle duality. Different experiments highlight different aspects of this duality. Wave phenomena such as interference and diffraction show light's wave nature, while some effects, like the photoelectric effect, strongly support the idea that light is made of discrete energy packets called photons. This question asks you to identify which phenomenon most directly demonstrates the particle nature of light.

Given Data / Assumptions:
• We consider several optical phenomena: diffraction, interference, photoelectric effect, polarisation, and refraction. • We assume standard textbook interpretations of these experiments. • We focus on which effect requires light to be quantised into photons to be explained correctly.

Concept / Approach:
The photoelectric effect involves the emission of electrons from a metal surface when light of sufficient frequency falls on it. Classical wave theory could not explain why there is a threshold frequency, why increasing intensity below this threshold does not cause emission, and why emitted electrons appear almost instantaneously. Einstein explained the effect by proposing that light comes in discrete quanta (photons) with energy E = h * f. Only if a single photon has enough energy to overcome the metal work function can it eject an electron. This explanation strongly supports the particle nature of light.

Step-by-Step Solution:
Step 1: Recall that diffraction and interference are classic wave phenomena, where waves overlap and produce patterns of maxima and minima. Step 2: Polarisation shows that light is a transverse wave with oscillations in specific directions, again supporting wave behaviour. Step 3: Refraction is the bending of light when it passes between media of different optical densities and can be explained using wavefronts or ray models. Step 4: The photoelectric effect, however, involves discrete emission of electrons depending on light frequency, not just intensity. Step 5: Einstein's photon model, where light energy is quantised as photons, successfully explains the existence of a threshold frequency and the immediate emission of electrons. Step 6: Therefore, the phenomenon that clearly demonstrates the particle nature of light is the photoelectric effect.

Verification / Alternative check:
Experiments show that increasing the intensity of light below a certain frequency does not cause photoemission, which contradicts classical wave expectations that more energy should eventually knock out electrons. However, if light is treated as photons, each with energy proportional to frequency, then light below threshold frequency has photons that individually lack the energy to liberate electrons, no matter how many are present. Only when photon energy exceeds the work function do electrons get emitted. This discrete, packet based behaviour is distinctly particle like.

Why Other Options Are Wrong:
Option a (Diffraction): Diffraction patterns arise from wave bending around obstacles and support the wave nature of light. Option b (Interference): The formation of interference fringes in double slit experiments demonstrates the superposition of waves. Option d (Polarisation): Polarisation shows that light has transverse oscillations and is a wave property, not a particle specific phenomenon. Option e (Refraction): Refraction can be described using wavefronts and Snell's law and does not by itself demand a particle model.

Common Pitfalls:
Learners sometimes think that any "special" optical effect must be evidence of particle behaviour, but many of the famous experiments (interference, diffraction, polarisation) actually confirm the wave model. The key is to identify which effect could not be explained by classical wave theory alone. The photoelectric effect is historically and conceptually the most important evidence for quantisation of light into photons and thus for its particle nature.

Final Answer:
The phenomenon that most clearly shows the particle nature of light is the Photoelectric effect.