frequency

Meaning: Frequency is the number of complete cycles of a repeating motion or wave per second, the inverse of the period. The source sets it, so a wave keeps its frequency when it enters a new medium. For sound it corresponds to pitch; for light, to color and photon energy. (In biology, allele frequency is a different idea: the fraction of a population's alleles of one type.)

Equation: f = 1/T and v = fλ, where f is frequency (Hz = s⁻¹), T is period (s), v is wave speed (m/s), and λ is wavelength (m). For photons, E = hf, with E in J and h = 6.63 × 10⁻³⁴ J·s.

On the MCAT: A wave crosses into a new medium (sound from air to water, light from air into glass) and you decide what changes: f stays fixed, while v and λ change in proportion. Frequency also drives the Doppler effect (observed f rises when source and detector approach each other), standing waves and resonance, beats (beat frequency = |f₁ − f₂|), and photon energy in spectroscopy and the photoelectric effect.

Common confusion: Students think a slower wave has a lower frequency. The medium sets v, the source sets f, and λ = v/f adjusts. Also do not confuse f with angular frequency ω = 2πf (rad/s).