number of oscillations or cycles per unit time
measured in hertz (Hz): 1 Hz = one cycle per second
inversely proportional to wavelength: f = v / lambda
audible sound: ~20 Hz to ~20,000 Hz
visible light: ~430 THz (red) to ~790 THz (violet)
radio frequencies span kHz to GHz, allocated by spectrum authorities
resonance occurs when a driving frequency matches a system's natural frequency
biological rhythms have characteristic frequencies: heartbeat (~1 Hz), brain waves (1-100 Hz)
sampling frequency must exceed twice the signal frequency (Nyquist theorem)