• Usually, when an object emits waves, the wavefronts spread out?symmetrically
  • If the wave source moves, the waves can become squashed together or stretched out

   

• Therefore, when a wave source moves relative to an observer there will be a change in the observed?frequency?and?wavelength

Wavefronts are even in a stationary object but are squashed in the direction of the moving wave source

• A moving object will cause the?wavelength, λ, (and frequency) of the waves to change:
  • The?wavelength?of the waves?in front?of the source?decreases?(λ – Δλ) and the?frequency?increases
  • The wavelength?behind?the source?increases?(λ + Δλ) and the?frequency decreases
  • This effect is known as the?Doppler effect?or?Doppler shift

   

• Note: Δλ means 'change in wavelength'

• Doppler shift can be calculated using the equation:

• Where:
  • λ = observed wavelength of the source in metres (m)
  • λ₀?= reference wavelength in metres (m)
  • Δλ = change in wavelength in metres (m)
  • v?= velocity of a galaxy in metres per seconds (m/s)
  • c?=?the speed of light?in metres per second (m/s)

   

• This means that the change in wavelength,?Δλ:

Δλ = λ – λ₀

• This equation can be used to calculate the velocity of a galaxy if its wavelength can be measured and compared to a reference wavelength
• Since the fractions have the same units on the numerator (top number) and denominator (bottom number), the Doppler shift has?no units