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David
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David

It was the study of the external photoelectric effect, which paved the way for a quantum theory of light! Discoveries made from experiments, beginning in 1887, showed that the external photoelectric effect had certain properties that could not be explained by classical, continuum theories in which light and all other kinds of electromagnetic radiation were considered to behave like waves. In classical theories, as the light shining on a metal becomes increasingly intense, the wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, these experiments… Read more »

Terry D
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Terry D

It is called QED, quantum electrodynamics. It is the relativistic quantum field theory that explains how photons mediate the force between charged particles and their anti-particles. It is based on Yang-Mills symmetry and was developed in particle form by Richard Feynman and in field form by Julian Schwinger.

Tuomo
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Tuomo

Everything is at the same time a particle and a wave. This was realized by de Broglie in 1924. It means that everything has both, particle like and wave like properties. The duality of light in this matter was discovered before theory of de Broglie. Quantum theory of light means just that light has particle like properties, which only depend on wavelength of the light, not the intensity of the light. Photoelectric effect is one example of this. In photoelectric effect the photons are absorbed by material which then releases electrons and thus creates current. If the wavelength of the… Read more »

Kes
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Kes

Quantum means quantity or amount. Light travels (at the speed of light) as mass-less packets of pure energy. The amount of energy of the photon depends upon its wavelength. Infrared (long wavelengths) is relatively weak while ultraviolet (shorter wavelengths) is stronger and more penetrating (sunburns through thin clouds, etc.). X-rays have even shorter wavelengths and can penetrate skin and bone while gamma rays have still shorter wavelengths and can penetrate lead to a certain extent. Photons can interact with electrons orbiting atoms (and molecules) forcing electrons to higher more energetic orbits. When the electrons return to rest orbits new photons… Read more »

Queenie
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Queenie

yeah, the quantum theory of light says that light exists as tiny packets, or particles, which are called photons.

Just as light is thought to have a dual nature, sometimes showing the characteristic of a wave, and sometimes that of a particle (photon), quantum theory attributes a similar dual wave-particle nature to subatomic particles.

Mario
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Mario

E=mc^2