Chapter One: An Acoustics Primer

6. What is wavelength? | Page 2

Doppler Shift

One particularly interesting frequency phenomenon involving wavelength is the Doppler effect or Doppler shift. You have no doubt seen movies in which a police siren or train whistle seems to drop in pitch as it passes the listener. In actuality, the wavelength of sound waves from a moving source is compressed ahead of the source and expanded behind it, creating the sensation of a higher, then lower, frequency for the listener than the source actually produces. This is the same phenomenon used by astronomers with light wavelengths to calculate the speed and distance of a receding star. The light wavelengths as stars move away from the observer are shifted toward the red end of the spectrum, hence the term redshift, which allowed Edwin Hubble to postulate, in 1929, that our universe was expanding.