Wave - Nami (English spelling)

Japanese: 波 - なみ（英語表記）wave

When a stone is thrown into a pond, circular ripples spread across the surface of the water. This phenomenon in which fluctuations that occur in one place are transmitted to other places one after another is called a wave or wave motion, and the medium that transmits the waves is called the medium. However, with any wave, the state of fluctuations in the medium is simply transmitted, and the medium itself does not move consistently. Some waves do not propagate through space, but vibrate with different amplitudes at each point. These are called standing waves or stationary waves. In contrast to standing waves, ordinary waves that propagate through space are called traveling waves to distinguish them. There are transverse waves and longitudinal waves. Transverse waves are waves in which the displacement direction of a point in the medium is perpendicular to the direction of wave propagation; electromagnetic waves are an example of this. Longitudinal waves are waves in which the displacement direction is the same as the direction of propagation; sound waves in the air are an example of this. Longitudinal waves are also called compressional waves. The surface created by connecting consecutive points of a medium that are in the same state at the same time, i.e., in the same phase, is called a wavefront. A wave with a flat wavefront is called a plane wave, and a wave with a spherical wavefront is called a spherical wave. In terms of light, a plane wave corresponds to a parallel beam of light, while a spherical wave corresponds to a beam of light emitted from a point light source that spreads out in all directions. The speed at which a wave front travels in a direction perpendicular to itself is called the phase velocity, and this is what is simply referred to as the speed of the wave. The distance between two adjacent wave fronts measured along the direction of wave travel is called the wavelength. If the frequency of a wave is ν and the wavelength is λ, then the phase velocity u is given by u = λν. Any shape is acceptable for a wave, but it is particularly important for each point in the medium to perform simple harmonic motion. Such a wave is called a sine wave. The differential equation that a wave satisfies is called the wave equation. (→ Ocean waves )

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

池に石を投げると，水の表面に円形の波紋が広がっていく。このように，1つの場所に起った変動が次々に他の場所に伝わる現象を波または波動といい，波を伝えるものを媒質という。しかし，どんな波でも，ただ媒質の変動の状態が伝えられるだけで，媒質そのものが一貫して移動するわけではない。波のなかには，空間を伝搬せず，各点で異なった振幅で振動する波もある。これを定在波または定常波という。定在波に対して，空間を伝搬する普通の波を進行波と呼んで区別する。波には横波と縦波がある。横波は媒質の点の変位方向が波の伝搬方向に対して垂直になっているもので，電磁波はこの例である。縦波は変位方向が伝搬方向と一致するもので，空気中の音波はこの例である。縦波は疎密波とも呼ばれる。同じ時刻に同じ状態，すなわち同じ位相にある媒質の連続した点を結んでできる面を波面という。波面が平面である波は平面波，球面である波は球面波と呼ばれる。光でいえば，平面波は平行光線に対応し，球面波は点光源から出て四方八方に広がっていく光線に対応する。波面が自分自身に垂直な方向に進む速さを位相速度といい，単に波の速さといえばこれをさす。隣合う2つの波面の間の距離を波の進行方向に沿ってはかった長さを波長という。波の振動数を ν ，波長を λ とすると，位相速度 u は u＝λν で与えられる。波の形はどんな形でもよいが，特に媒質の各点が単振動をする場合が重要である。このような波を正弦波という。波が満足する微分方程式を波動方程式という。 (→波浪 ) 　

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