|Google Doodle, 2/22/2012|
Frequency (or more precisely temporal frequency) is a fundamental physical idea. All kinds of objects vibrate , more or less, periodically - leaves flutter in the wind, hollow tubes whistle, trees sway, balls bounce. etc. etc. Surface waves move water up and down and move foward and backward. The frequency spectrum of a time-domain function is a unique transformation (the Fourier transform). Time and frequency are dual variables related through an analytic duality. Here is a list of dualities. The one of interest here is Pontryagin duality .