Frequency
Definition
How many wave cycles pass a point every second. Higher frequency means more cycles per second.
Symbol: · Unit: hertz (Hz)
Technical definition
The number of oscillations per unit time, f = 1/T, measured in hertz. For waves, frequency is set by the source and does not change when the wave enters a new medium — only speed and wavelength change.
Example
A guitar string vibrating 440 times per second produces the A above middle C. Human hearing spans roughly 20 Hz to 20,000 Hz — below that is infrasound, above it ultrasound.
Frequency is not speed. When a wave moves from air into water, its speed changes but its frequency stays exactly the same — the wavelength adjusts instead (λ = v/f). Students who think frequency changes with the medium get refraction and the Doppler effect backwards: the source sets the frequency, the medium sets the speed.