# Translations:Introduction - Chapter 1/12/en

The phenomena perceived by our eyes as light and by our ears as sound are propagated as wave motion. That motion occurs not on a 2D surface such as the plane formed by the top of a still pond but in 3D space. Many of the properties of wave motion can be understood by studying the familiar waves traveling in a body of water. Water waves produced by a dropped stone move out in circular rings at a constant speed. That wave speed is called the velocity of propagation and is denoted by *v*. The waves themselves have crests and troughs - points at which the water level is elevated and points at which the level is depressed. The water surface undulates rhythmically between crests and troughs. The distance between successive crests or between successive troughs is called the *wavelength* and usually is denoted by the Greek letter . As the waves travel past a fixed point on the surface of water, they cause a vertical up-and-down motion of the water at that given point. Such up-and-down motion repeats itself in time in a periodic manner. The number of times per second that the up-and-down motion repeats itself is called the *frequency* of the wave and is denoted by *f*. In what follows, we shall examine in mathematical detail three fundamental aspects of wave motion: