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What evidence showed that photons have discrete amounts of energy?
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When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.
The evidence that photons possess discrete amounts of energy—defined by the relationship —emerged from several key developments in early 20th-century physics:
- Blackbody Radiation: In 1900, Max Planck proposed that electromagnetic radiation is emitted and absorbed in discrete units, or quanta, to explain the spectrum of blackbody radiation.61
- Photoelectric Effect: In 1905, Albert Einstein extended this concept by proposing that light itself consists of discrete energy packets (later called photons). This explained why the kinetic energy of ejected electrons depends on the light's frequency rather than its intensity. This theory was later confirmed by Robert Millikan in 1916.234
- Compton Effect: In 1923, Arthur Holly Compton demonstrated that X-rays scatter off electrons as if they were particles with specific energy and momentum, providing direct evidence for the corpuscular nature of radiation.51
These findings established that light behaves as both a wave and a particle, with its energy strictly proportional to its frequency via Planck’s constant.567