9.5.2 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:

• 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)

• • Received frequency, f_r = 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 3: Rearrange for speed v, and calculate

= 1.5 × 10⁶ m s^–1

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