Every wave that will ever appear on your paper is described by four numbers. Wavelength $\lambda$ (said lambda) runs ALONG the wave — metres from one crest to the next. Amplitude stands ACROSS it — metres from rest position out to a crest. Period $T$ is the seconds one full cycle takes; frequency $f$ counts cycles each second, in hertz — 1 Hz is one cycle per second.

Period and frequency are the same fact read in opposite directions, and the bridge between them is one flip: $T = \dfrac{1}{f}$ (read aloud: T equals one over f). Five cycles a second means a fifth of a second each; no other formula on this page is that polite.

This lesson is the reading skill: which of the four numbers is the sentence handing you? The units point the way — Hz counts, bare seconds time, and metres measure either along the wave (wavelength) or out from rest (amplitude). Read WHERE the metres run.