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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.
The wave nature of light is demonstrated by phenomena that can only be explained by waves:
These experiments conclusively show that light propagates as a wave, with properties such as wavelength, frequency, amplitude, and phase.
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=λhc
and momentum:
p=λh=cE
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