A-level Physics (Advancing Physics)/Doppler Effect/Worked Solutions

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1. M31 (the Andromeda galaxy) is approaching us at about 120kms−1. What is its red-shift?

The minus sign is important! Andromeda is blue-shifted!

2. Some light from M31 reaches us with a wavelength of 590 nm. What is its wavelength, relative to M31?

3. Some light has a wavelength, relative to M31, of 480 nm. What is its wavelength, relative to us?

4. A quasar emits electromagnetic radiation at a wavelength of 121.6 nm. If, relative to us, this wavelength is red-shifted 0.2 nm, what is the velocity of recession of the quasar?

However, this is about as high a velocity as we can use the classical Doppler effect for.