Practise explaining redshift for receding stars, identifying changes in wavelength and frequency, and applying the Doppler effect to observed light.
The star in (b) is in a distant galaxy. A spectral line in the light from this galaxy is known to have a wavelength of 486 nm . This spectral line in the light from the galaxy observed on the Earth has a wavelength of 492 nm .
Explain why the wavelength observed on the Earth is different from the wavelength that the galaxy is known to have emitted.
galaxy is moving away (from the Earth)
wavelength (of light from the galaxy) increased by the Doppler effect / due to redshift