The Doppler Effect of Light

• What is the Doppler shift equation for a light-emitting non-relativistic source?

  ∆f/f = ∆λ/λ ≈ ∆v/c
• What does ∆f represent in the Doppler shift equation?

  Change in frequency (Hz)
• What does f represent in the Doppler shift equation?

  Reference frequency (frequency of the source) (Hz)
• What does ∆λ represent in the Doppler shift equation?

  Change in wavelength (m)
• What does λ represent in the Doppler shift equation?

  Reference wavelength (wavelength of the source) (m)
• What does ∆v represent in the Doppler shift equation?

  Relative velocity of the source and observer (m/s)

  
• What does c represent in the Doppler shift equation?

  Speed of light (m/s)
• How is the change in wavelength ∆λ defined?

  ∆λ = λ' − λ
• What is the formula for relative speed between the source and observer?

  ∆v = vs − vo
• What can be assumed about the observer's velocity in many cases?

  It can be assumed to be stationary (vo = 0)
• What does the relative speed become when the observer is stationary?

  ∆v = vs = v
• What is the fractional change in wavelength called?

  Redshift
• How is redshift defined in terms of wavelength?

  z = ∆λ/λ = −v/c
• How is redshift defined in terms of frequency?

  z = −∆f/f = v/c
• What happens to wavelength and frequency when the source moves away from the observer?

  Wavelength increases, frequency decreases

  
• What is observed when a star approaches the Earth?

  Blueshift is observed (negative z)
• What is the relative velocity v when a star approaches the Earth?

  Relative velocity v is positive
• What is the change in wavelength ∆λ when a star approaches the Earth?

  The change in wavelength ∆λ is negative
• What is observed when a star recedes from the Earth?

  Redshift is observed (positive z)
• What is the relative velocity v when a star recedes from the Earth?

  Relative velocity v is negative
• What is the change in wavelength ∆λ when a star recedes from the Earth?

  The change in wavelength ∆λ is positive
• What are the steps to calculate the speed of a star moving away from Earth using the Doppler effect?

  1. List known quantities (unshifted and shifted wavelengths).
  2. Calculate change in wavelength: ∆λ = λ' - λ.
  3. Use Doppler equation: v = c∆λ/λ.
  4. Substitute values to find v.
• What is the shifted wavelength of the light from the distant star?

  608 nm
• What is the speed of light used in the calculations?

  3.0 × 10⁸ m/s
• What is the calculated speed at which the star is traveling away from the Earth?

  1.16 × 10⁸ m/s
• Which side of the galaxy is moving towards the Earth based on the observed wavelengths?

  The right-hand side is moving towards Earth
• What is the observed wavelength from the left-hand side of the galaxy?

  656.44 nm
• What is the observed wavelength from the right-hand side of the galaxy?

  656.12 nm
• What is the reference wavelength measured on Earth?

  656.28 nm
• What is the speed of light in the visible light spectrum?

  Red light has longer wavelength
• How does blue light compare to red light in terms of frequency?

  Blue light has higher frequency
• What is the relationship between wavelength and frequency in the visible light spectrum?

  Red light has longer wavelength, lower frequency