Waves

Watch a duck on a pond: it bobs up and down as a wave passes but never drifts toward shore. That is the whole idea of a wave. Each bit of water (or string, or air) oscillates about a fixed position, and it's that oscillation passed from neighbor to neighbor that carries energy across the pond, not any actual transport of water. On a drawing of a wave, amplitude is the height from the resting line to a crest (or trough), and it measures the energy the wave carries: taller wave, more energy, same speed. Wavelength is the distance between two consecutive crests. Period is the time for one full oscillation at a single point, and frequency is how many oscillations happen per second, so frequency = 1/period. Speed, frequency and wavelength are tied together by speed = frequency x wavelength.

When two waves meet, they add up: at any point, their heights simply combine. Line up two crests and you get constructive interference, a taller crest; line up a crest with a trough and you get destructive interference, flattening toward nothing. After they pass, each wave continues on unchanged, as if the meeting never happened.

Speed is set by the medium, not by amplitude, so making a wave taller changes its energy but not how fast it travels. But speed does change when a wave crosses into a new medium, and since the source fixes the frequency, wavelength must adjust: slower speed with the same frequency means a shorter wavelength. This links waves to the rest of physical science's view of energy transfer without mass transfer.