Light is not so easy to understand as a wave, which is why there are multiple sections of this book devoted to it. Water waves and sound waves are examples of mechanical waves - waves that propagate through a material medium. The medium through which this disturbance propagates is the air. The air under the book bumps the air on the edges of the book, which bumps the air next to it, which bumps the air next to it, and so on. The rise and fall of pressure is like the rise and fall of the surface of the pool in the previous example. The air pressure in that rapidly decreasing gap rises above normal and then rebounds. The air between the book and the table is squeezed out in a fraction of a second. The medium through which this disturbance propagates is the surface of the water. The disturbance spreads outward, transmits, or propagates. The disturbed water at the point of impact disturbs the water next to it, which in turn disturbs the water next to it, which disturbs the water next to it, and so on. It is higher than normal in some places and lower than normal in others. The most important kinds of waves for humans are the waves we use to sense the world around us: sound and light. The first example that comes to mind when most people hear the word wave are the kinds of waves that one sees on the surface of a body of water: deep water waves in the ocean or ripples in a puddle. Let's list a few key examples of wave phenomena and then connect them to this definition. Media is the plural form (although mediums is prefered by some people). Transmission lines are the medium of alternating current electric power. Cell membranes are the medium of nerve impulses. The Earth is the medium of seismic waves (earthquake waves). The electric and magnetic fields are the medium of light. Air is the medium through which we hear sound waves. A medium is the substance through which a wave can propagate.The noun form of the word is propagation. Synonyms for propagate include spread, transmit, communicate, and broadcast. To propagate, in the sense used in this definition, is to transmit the influence of something in a particular direction.a change in field strength like electric field strength, magnetic field strength, or gravitational field strength.a change in an intensive property like pressure, density, or temperature.a change in a kinematic variable like position, velocity, or acceleration.A disturbance, in the sense used in this definition, is a change from the current state of a measurable quantity at some location.There are three words in that definition that may need unpacking: disturbance, propagate, and medium. A wave is a disturbance that propagates through a medium.
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