Caption: A generic classical Doppler effect image for a point source that generates spherical waves when at rest in the medium

Features:

1. Recall the Doppler effect is the shift in frequency of a wave phenomenon from a reference frequency and that shift depends on the motion of an observer of the wave phenomenon. The Doppler effect occurs for all wave phenomenon.

   However, the classical Doppler effect formulae and behavior are different from the (extreme limit) relativistic Doppler effect which is experienced by electromagnetic radiation (EMR) (in vacuum: i.e., with NO medium) since EMR is always moving at the vacuum light speed c = 2.99792458*10**8 m/s ≅ 3*10**8 m/s = 3*10**5 km/s ≅ 1 ft/ns, and so is always extremely relativistic.

2. In the image, the point source is moving to the left and the expanding waves are shown as distorted light and dark spheres in cross section.

3. Those waves emitted to the left have their wavelength decreased and their frequency increased---in the jargon of astrophysics, they are blueshifted.

   Those waves emitted to the right have their wavelength increased and their frequency decreased---in the jargon of astrophysics, they are redshifted

   Note a blueshift is can be considered a negative redshift and often is in astronomy.

   Other directions give intermediate cases of Doppler shifting. We will NOT explicate other directions, except to note that there is NO classcial Doppler shift for motion perpendicular to the direction of wave propagation.

4. The classical Doppler effect formulae are derived and somewhat explicated in Waves file: doppler_effect_classical_derivation.html.