Frequency

• Frequency is the number of occurrences of a repeating event per unit time, or the number of complete waves to pass a point every second.
• The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency.

• The below figure shows the relation between frequency and wavelength.

Angular frequency

• Angular frequency, usually denoted by the Greek letter ω (omega), is defined as the rate of change of angular displacement, θ, (during rotation), or as the rate of change of the argument to the sine function:

• Angular frequency is commonly measured in radians per second (rad/s).

Spatial frequency

• Angular frequency, usually denoted by the Greek letter ω (omega), is defined as the rate of change of angular displacement, θ, (during rotation), or as the rate of change of the argument to the sine function:

• Wavenumber, k, is the spatial frequency analogue of angular temporal frequency and is measured in radians per meter.
• In the case of more than one spatial dimension, wavenumber is a vector quantity.