Answer:
4) measure the shift in the emission spectrum of a star.
Explanation:
Doppler effect occurs when a source emitting waves is moving relative to an observer (or when the observer is moving relative to the source). When this occurs, the frequency (and the wavelength) of the waves emitted by the source appears shifted to the observer.
In astronomy, this effect occurs with stars and galaxies. In fact, stars (and galaxies as well) move relative to us. Since stars are source of visible light (which is a form of electromagnetic radiation, so it is a form of wave), the frequency and the wavelength of the light we observe from a star is shifted. In particular, two situations are possible:
- When a star is moving away from us, its wavelength appears to be increased compared to the original wavelength: therefore, the spectrum of visible light shifts towards the red (which is the color with longer wavelength), and this is called red-shift
- When a star is moving towards us, its wavelength appears to be decreased compared to its original wavelength: therefore, the spectrum of visible light shifts towards the blue (shorter wavelength), and this is called blue-shift
