Wave-Particle Duality

By the early 20th century, physics faced a remarkable paradox. Light, long established as a wave (demonstrated by interference and diffraction), also behaved as a stream of particles (demonstrated by the photoelectric effect). Meanwhile, electrons, long established as particles, turned out to exhibit wave behaviour. This dual nature of matter and radiation — wave-particle duality — is one of the cornerstones of quantum physics.

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Light as a Wave

The wave nature of light is demonstrated by phenomena that can only be explained by waves:

• Interference: Young's double-slit experiment produces alternating bright and dark fringes — a hallmark of wave superposition
• Diffraction: Light spreads when passing through narrow slits or around obstacles
• Polarisation: Light can be polarised, which is only possible for transverse waves

These experiments conclusively show that light propagates as a wave, with properties such as wavelength, frequency, amplitude, and phase.

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Light as a Particle

The photoelectric effect (and other experiments such as the Compton effect) revealed that light also behaves as a stream of discrete particles — photons. Each photon carries energy:

$E = hf = \frac{hc}{\lambda}$E=hf=λhc​

and momentum:

$p = \frac{h}{\lambda} = \frac{E}{c}$p=λh​=cE​