How can radiation show particle behavior when it interacts with matter?

Radiation exhibits particle-like behavior when it interacts with matter, a phenomenon central to wave-particle duality. While radiation propagates through space as a wave, its interactions with matter reveal its discrete, corpuscular nature.1

Two primary processes demonstrate this particle behavior:

• The Photoelectric Effect: When light strikes a metal surface, it can eject electrons. Classical wave theory fails to explain this because the kinetic energy of the emitted electrons depends on the frequency of the light rather than its intensity. Albert Einstein resolved this by proposing that light consists of discrete packets of energy called photons, where each photon carries energy $E=hf$.23
• Compton Scattering: When high-energy X-rays or gamma rays collide with electrons, they behave like particles with momentum and energy. Arthur Holly Compton demonstrated that these collisions follow the laws of relativistic mechanics, resulting in a measurable shift in the photon's wavelength based on the scattering angle.453

These interactions confirm that radiation can act as a collection of particles, providing essential evidence for modern quantum theory.13