The particle nature of light is demonstrated by the

Step 1: Analyze the question The question asks which phenomenon shows that light behaves as a particle (not a wave).

Step 2: Explain the concept Light shows wave-particle duality:

• Wave nature is shown by interference, diffraction, polarization, and refraction.
• Particle nature (photons) is shown by the photoelectric effect and the Compton effect.

In the photoelectric effect, light shining on a metal surface ejects electrons only if the frequency is at or above a threshold frequency, no matter how bright the light is. Wave theory cannot explain this. Einstein explained it by saying light travels in packets of energy called photons, each with energy:

E = hf

Here h is Planck's constant and f is the frequency. An electron is released only if a single photon has enough energy to overcome the work function:

hf = φ + KE(max)

Step 3: Check each option

• A. Photoelectric effect: Correct. It needs photons (particles) to be explained.
• B. Speed of light: This is a measured constant and does not show either nature.
• C. Colours of light: Colours correspond to different wavelengths or frequencies and are explained by wave properties (e.g., dispersion).
• D. Diffraction of light: This is a classic wave property.

Step 4: Answer The particle nature of light is demonstrated by the photoelectric effect, which is option A.

Exam tip: Remember "Photoelectric = Particle" and "Diffraction/Interference = Wave". Whenever you see photoelectric effect in a "particle nature" question, choose it.