Watch waves cross a pond and you can measure three things: how long each wave is (the wavelength, written $\lambda$ — the Greek letter lambda), how many pass per second (the frequency $f$, in hertz — 2 Hz just means twice a second), and how fast the whole parade moves (the wave speed $v$, in metres per second). One formula ties them: $v = f\lambda$ (read aloud: v equals f lambda). It is honest bookkeeping: $f$ waves leave each second, each one $\lambda$ metres long, so the front edge gains $f \times \lambda$ metres every second. Its little cousin $T = \tfrac{1}{f}$ (T equals one over f) gives the period $T$, in seconds — the time one full cycle takes — just the frequency flipped over.

And here is the wonderful part: light plays by the same rule. Light waves are simply far too small for ponds — green light's wavelength is about 500 nanometres, and a nanometre is a billionth of a metre; roughly two thousand of those waves fit across one millimetre. Same formula, tinier ruler. This lesson practises the rule on waves you can actually see.