A group of waves consists of several crests separated by troughs. The height of the waves is called the amplitude, the distance between successive wave crests is known as the wavelength, and the time between successive wave crests is the period. Waves are classified into types based on their periods. They range from ripples, which have periods of less than 0.5 seconds, up to tsunamis and tides, whose periods are measured in minutes and hours (their wavelengths range from hundreds to thousands of miles).
In between these extremes are chop and swell—the most familiar types of surface waves. Ocean waves behave like light rays: they are reflected or refracted by obstacles they encounter, such as islands. When different wave groups meet, they interfere—adding to, or canceling, each other.