Quantum Mechanics (Encyclopedia of Science)
Quantum mechanics is a method of studying the natural world based on the concept that waves of energy also have certain properties normally associated with matter, and that matter sometimes has properties that we usually associate with energy. For example, physicists normally talk about light as if it were some form of wave traveling through space. Many properties of lightuch as reflection and refractionan be understood if we think of light as waves bouncing off an object or passing through the object.
But some optical (light) phenomena cannot be explained by thinking of light as if it traveled in waves. One can only understand these phenomena by imagining tiny discrete particles of light somewhat similar to atoms. These tiny particles of light are known as photons. Photons are often described as quanta (the plural of quantum) of light. The term quantum comes from the Latin word for "how much." A quantum, or photon, of light, then, tells how much light energy there is in a "package" or "atom" of light.
The fact that waves sometimes act like matter and waves sometimes acts like waves is now known as the principle of duality. The term duality means that many phenomena have two different faces, depending on the circumstances in which they are being studied.
Macroscopic and submicroscopic properties
Until the 1920s,...
(The entire section is 786 words.)
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