Illustration demonstrating a sine wave by a revolving circle. Sine waves (grey) represent the behaviour of a simple oscillator. As the circle rotates, the arrows trace out a sine wave. The maximum amplitude of the wave is the same as the radius of the circle. The angle that the arrow makes with a horizontal plane (not shown) is known as the phase angle. The sine of this angle determines the height of the wave at any point. Sine waves are periodic and repeat after a complete 360 degree revolution (2 pi radians) of the unit circle. These functions often appear in mathematics, physics and engineering.