It occurs because the galaxy is moving away from us and we are moving away from it. This shift towards the red end of the spectrum is the effect known as red-shift. If we then compared this to an absorption spectrum produced using light from a star which is the same size as the Sun in another galaxy, we would see that the position of the dark lines would shift towards the red end of the spectrum as shown in the diagram below. If we looked at the absorption spectrum for light on Earth emitted by our closest star, the Sun, we would observe dark lines in specific positions. Dark lines would be seen in the spectrum where light at that frequency and wavelength has been absorbed. A spectroscope can be used to view the absorption spectrum for any light emitting object. Red-shiftĪ spectrum shows the seven colours making up visible light. Red light has a long wavelength and lower frequency, whereas the light at the blue/violet end of the visible spectrum has a shorter wavelength and higher frequency. When the source of light is moving away from you, the light waves will be stretched out with a longer wavelength and lower frequency. When a source of light is travelling towards you, the light waves will be closer together with a shorter wavelength and higher frequency. The Doppler effect is also observed with light waves. Once the ambulance has passed you and is travelling away, the frequency of the waves decreases and the sound is lower in pitch. There is no need to convert nanometres to metres as units cancel top and bottom.As shown in the diagram, as the ambulance travels towards you the frequency of the waves is increased and the sound will be higher in pitch. So if a distant galaxy emits a characteristic spectral line of 91 nm ( ultraviolet light at the 'Lyman limit') but when observed on Earth it appears to be 640 nm (red) we can calculate the red shift using this equation: The following equation is used to calculate redshift: This is because galaxies are receding (moving away) at such high speeds that relativistic effects need to be considered in calculations. The Doppler equation used for sound calculations cannot be used in this situation. Redshift is also the name of the factor z indicating the relative change in wavelength due to the Doppler shift for a receding galaxy. The upper diagram shows the absorption spectrum from a stationary galaxy with one wavelength of light and no redshift. Lower diagram shows spectrum of light redshifted from a distant galaxy moving away from the Earth This indicated the stars were moving away from Earth (just as the sound of a siren moving away from you has a decreased frequency and increased wavelength).Īs the light was shifted towards the red end of the spectrum (lower frequency/longer wavelength) this phenomenon was termed 'redshift'. The same shifts in frequency and wavelength are also observed for light coming from stars in distant galaxies.īy comparing the light from distant stars with the spectrum of light from our Sun it was noticed that the spectra from distant stars had a slightly decreased frequency and slightly increased wavelength.
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