IB DP Physics: HL复习笔记9.5.2 Uses of The Doppler Effect

Uses of The Doppler Effect

  • The Doppler effect is important in many key areas of science including:
    • Radar readings for moving objects
    • Measuring the rate of blood flow within a patient
    • Finding planet orbits around distant stars
    • Mapping the expansion of the universe

Redshift of EM Radiation

  • On Earth, the Doppler effect of sound can be easily observed when sound waves move past an observer at a notable speed
  • In space, the Doppler effect of light can be observed when spectra of distant stars and galaxies are observed, this is known as:
    • Redshift if the object is moving away from the Earth, or
    • Blueshift if the object is moving towards the Earth
  • Redshift is defined as:
The fractional increase in wavelength (or decrease in frequency) due to the source and observer receding from each other
  • For non-relativistic galaxies, Doppler redshift can be calculated using:
  • Where:
    • Δλ = shift in wavelength (m)
    • λ = wavelength emitted from the source (m)
    • Δf = shift in frequency (Hz)
    • f = frequency emitted from the source (Hz)
    • v = speed of recession (m s-1)
    • c = speed of light in a vacuum (m s-1)

Worked Example

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.

Calculate the speed of the distant galaxy in relation to the Earth. Determine whether it is moving towards or away from the Earth.

Step 1: Write down the known quantities
    • Received frequency, fr = 4.547 × 1014 Hz
    • Original frequency, f0 = 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 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

An Expanding Universe

  • After the discovery of Doppler redshift, astronomers began to realize that almost all the galaxies in the universe are receding
  • This led to the idea that the space between the Earth and the galaxies must be expanding
  • This expansion stretches out the light waves as they travel through space, shifting them towards the red end of the spectrum
  • The more red-shifted the light from a galaxy is, the faster the galaxy is moving away from Earth


  • The expansion of the universe can be compared to dots on an inflating balloon
    • As the balloon is inflated, the dots all move away from each other
    • In the same way as the rubber stretches when the balloon is inflated, space itself is stretching out between galaxies
    • Just like the dots, the galaxies move away from each other, however, they themselves do not move