What is the difference between a red and blue shift?
The visible light spectrum spans a wavelength range from about 400 nanometers (1 nanometer = 1 billionth of a meter, abbreviated nm) to about 750 nm. The 450 nm light appears to your eyes as a deep blue color; the 750 nm light appears red. A “red shift” means that a spectral line (a narrow range of wavelengths of light) that is normally observed at say 500 nm is “shifted” towards the “redder” end of the spectrum to a longer wavelength of say 510 nm. Conversely, a “blue shift” means that the same spectral line is observed to be shifted towards the blue end of the spectrum, say at 490 nm. Red or blue shifts observed in spectral lines are typically caused by motion of the light source with respect to the observer. When the light source is moving away from the observer, it appears red-shifted; when the source is moving towards the observer it appears blue shifted. This change in the observed wavelength of a moving source is known as the “Doppler Effect” and it occurs for all kinds of waves
