We're just having a discussion at home and would like to clarify a point. The apparent shift of light toward the red when the emission source is moving away from us, or toward the blue when the emitter is moving toward us, is called the Doppler shift. Let us take a minute to try to explain what this is.
Light consists of fluctuations, or waves, of the electromagnetic field.
The different wavelengths of light are what the human eye sees as different colors. The longest wavelengths appear in the red end of the spectrum and the shortest appear in the blue end. Now imagine a source of light at a constant distance from us, emitting waves of light at a constant wavelength. Obviously, the wavelength of the waves we receive will be the same constant wavelength at which they are emitted by the source. Suppose now that the source starts moving directly toward us.