- 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)