Number of waves passing a point per second is called what?

• A. Wavelength
• B. Speed
• C. Velocity
• D. Frequency

Answer: (D)

Frequency is the rate at which wave cycles pass a point in time. It tells you how many crests (or troughs) cross a fixed point every second, and its unit is hertz (1 Hz = 1 cycle per second). This is why the correct term is frequency. Wavelength describes the distance between successive crests, not how often they pass a point. Speed (or velocity) describes how fast the wave travels through space (and velocity includes direction), and the relationship between these properties is v = fλ, so frequency connects speed and wavelength. For example, if five waves pass a point each second, the frequency is 5 Hz.