Final answer:
Red shift and blue shift describe how the observed wavelength of electromagnetic radiation lengthens when the source moves away and shortens when the source moves toward the observer, which are key parts of the relativistic Doppler effect.
Step-by-step explanation:
The terms "red shift" and "blue shift" refer to changes in the observed wavelength of electromagnetic radiation due to the motion of the source relative to the observer. Specifically, when the source of the radiation is moving away from the observer, the observed wavelength is longer, which is known as a red shift. Conversely, when the source is moving towards the observer, the observed wavelength is shorter, referred to as a blue shift. These phenomena are aspects of the relativistic Doppler effect, which takes into account the effects of the relative velocity of the source and observer as well as the finite speed of light.
According to the relativistic Doppler effect, if the relative velocity is zero, there would be no shift in the observed wavelength which is the expected result. Additionally, the relativistic Doppler effect is consistent with the classical Doppler effect in that the observed wavelength is indeed longer when the source moves away from the observer, just as the classical Doppler effect predicts a lower frequency (thus longer wavelength) for receding sources.