The apparent change in wavelength or frequency of the radiation from a source due to its relative motion away from or toward the observer
The fractional increase in wavelength (or decrease in frequency) due to the source and observer receding from each other
The spectra below show dark absorption lines against a continuous visible spectrum.
A particle line in the spectrum of light from a source in the laboratory has a frequency of 4.570 × 1014 Hz. The same line in the spectrum of light from a distant galaxy has a frequency of 4.547 × 1014 Hz. What speed is the distance galaxy moving in relation to the Earth? Is it moving towards or away from the Earth?
Step 1: Write down the known quantities
Emitted frequency, f = 4.570 × 1014 Hz
Shift in frequency, Δf = (4.547 – 4.570) × 1014 = –2.3 × 1012 Hz
Speed of light, c = 3.0 × 108 m s–1
Step 2: Write down the Doppler redshift equation
Step 3: Rearrange for speed v, and calculate
Step 4: Write a concluding sentence
The observed frequency is less than the emitted frequency (the light from a laboratory source), therefore, the source is receding, or moving away, from the Earth at 1.5 × 106 m s–1
In your exam, be sure to emphasise that redshift means the wavelength of spectral lines increases towards the red end of the spectrum, do not say that the spectral lines become red, as this is incorrect.
A balloon inflating is similar to the stretching of the space between galaxies