The document discusses the Doppler effect, which is a change in frequency or wavelength of a wave for an observer moving relative to the wave source. It was named after Austrian physicist Christian Doppler. The Doppler effect can occur when either the source of waves or observer is in motion. When the source is moving toward the observer, the observed frequency is higher than the source frequency, and when the source is moving away the observed frequency is lower. Diagrams show how the wavelength changes from the perspective of stationary versus moving observers. An equation is also derived relating the source speed, observed frequency, and source frequency.