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<p class="head">Photon. Wave-particle duality</p>
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<p>The tiniest packet of energy that can be emitted or absorbed in the form of radiation is called <a class="def">quantum</a>. This is smallest unit of radiation energy possible in nature.
<p><a class="def">Photon</a> is a quantum of <a class="black" href="https://cvertan.github.io/physics4dh.github.io/12_EM_radiation.html" target="frameterms">electromagnetic radiation</a>. <a class="black" href="https://cvertan.github.io/physics4dh.github.io/12_EM_radiation.html" target="frameterms">Light</a> (that is electromagnetic radiation) propagates as a flux of photons.</p>
<p>Introduction of the concept of ‘quantum’ by German theoretical physicist Max Plank in 1900 in the contest of ‘light’ was crucial. According to Planck's quantum theory, light (or more broadly, electromagnetic radiation) <a class="black" href="https://cvertan.github.io/physics4dh.github.io/20_Energy.html" target="frameterms">energy</a> can be emitted or absorbed in discrete quantities only. This tiniest quantity was called ‘quantum’. The word ‘quantum’ comes from Latin, meaning ‘an amount’ or ‘how much?’ The plural form is quanta. The notion of a photon was introduced in 1926 by the American chemist Gilbert New-ton Lewis. ‘Photon’ comes from the ancient Greek word meaning ‘light’.</p>
<p>The introduction of the concept of ‘quantum’ has marked the beginning of the development of quantum mechanics, later quantum theory of information, quantum computing and other fields of science.</p>
<p>To understand the concept of ‘photon’, it is important to recognize the dual nature of the photon. Namely, a photon can be considered as both <a class="black" href="https://cvertan.github.io/physics4dh.github.io/15_Wave.html" target="frameterms">wave</a> and <a class="black" href="https://cvertan.github.io/physics4dh.github.io/14_Particle.html" target="frameterms">particle</a>. For example, when light is travelling through space it exhibits wave-like behavior. When light is <a class="black" href="https://cvertan.github.io/physics4dh.github.io/8_Emission.html" target="frameterms">emitted</a> or <a class="black" href="https://cvertan.github.io/physics4dh.github.io/7_Absorption.html" target="frameterms">absorbed</a> by material, it exhibits particle-like behavior. So, light shows properties of both waves and particles, depending on the circumstances and the effect being observed.</p>
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<div class="desc">Wave–particle duality concept of the photon</div>
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<img src="Figures/P1_wAVE PARTICLE DUALITY.jpg" >
<div class="desc"> Properties of the object from different points of view © <a href="https://www.researchgate.net/post/Why-is-wave-particle-duality-important" target="_blank"> Author - Nicolae Sfetcu </a> </div>
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<p>Generally, <a class="def">wave–particle duality</a> is the concept in quantum mechanics that every quantum entity may be described as either a particle or a wave. It expresses the inability of the classical concepts ‘particle’ or ‘wave’ to fully describe the behavior of quantum-scale objects (scale of atoms, 10<a class ="super">-12</a>-10<a class ="super">-10</a> m). We can consider wave–particle duality concept of the photon as an epistemological phenomenon, that must be just accepted for now. Also, one can consider wave–particle duality like views on the same object from different points: one sees different properties of the same object, depending on the observation circumstances. </p>
<p>Physicists and mathematicians use the representation of a photon as a particle or a wave to build physical and mathematical theoretical models of phenomena involving light that we observe in nature, and to provide a reasoned explanation for these phenomena.</p>
<p>In quantum mechanics theory, in particle-like aspect, the photon is considered as a massless particle with certain <a class="black" href="https://cvertan.github.io/physics4dh.github.io/7_Absorption.html" target="frameterms">energy</a>. On the other hand, in wave-like aspect, the photon is considered as a wave having corresponding wave characteristics (as <a class="black" href="https://cvertan.github.io/physics4dh.github.io/15_Wave.html" target="frameterms">wavelength</a>, <a class="black" href="https://cvertan.github.io/physics4dh.github.io/15_Wave.html" target="frameterms">frequency</a>, etc.). When a photon is detected by a measuring instrument, it is registered as a single, particulate unit. However, the probability of detecting a photon is calculated by equations that describe waves. </p>
<p>Photons always move at the <a class="def">speed of light in vacuum (300,000 km/s)</a> and theoretically can never stop and be at rest. This is because the speed of light is the absolute speed limit in the universe and any particle traveling slower than the speed of light would have a mass. Photons are massless because they have no rest mass. This means that they cannot be measured by weighing them, since any mass they gain from their energy is always in motion. Photons travel at the speed of light, and their energy is entirely in their momentum, not their mass. This means that a photon can never be at rest, and thus can never have a rest mass. </p>
<p>Photons carry energy but have no mass. </p>
<p>Photons are produced by <a class="black" href="https://cvertan.github.io/physics4dh.github.io/5_Atom.html" target="frameterms">atoms</a> and interact in three ways with the atoms: the <a class="black" href="https://cvertan.github.io/physics4dh.github.io/7_Absorption.html" target="frameterms">absorption</a>, <a class="black" href="https://cvertan.github.io/physics4dh.github.io/8_Emission.html" target="frameterms">spontaneous emission</a> and <a class="black" href="https://cvertan.github.io/physics4dh.github.io/8_Emission.html" target="frameterms">stimulated emission</a>. </p>
<p class="important">More on the subject:
<a href="physics light photon particle wave massless link" href="https://astronomy.com/magazine/ask-astro/2022/05/ask-astro-how-can-a-photon-be-massless" target="_blank">How can a photon be massless</a></p>
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