Light has a speed limit. It's 300,000,000m/s in a vacuum. Sound has a speed limit too but it's a moveable one. Heat the air up, and sound goes faster. If you imagine something that makes light or sound waves like a pebble dropped into a pond, the waves are like the ripples spreading out from the centre. What happens if the object moves as it makes the waves. The speed limit means that the waves cannot move away from the object any faster or slower. So the waves given off in the same direction as the object is moving get bunched up. This makes them a shorter wavelength and a higher frequency. Those given off behind get spread out. This makes them have a longer wavelength and a lower frequency.
If the object is coming towards you and making sound, the pitch will be higher. If it's making light the colour will be bluer.
If the object is going away from you and making sound the pitch will be lower. If it's making light its colour will be redder.
This is why high speed things like cars and planes make a 'neeow' sound as they race past you. Sound travels at about 340m/s. So a car travelling at 30m/s will increase the pitch considerably. You don't notice a colour change because the change is so tiny on something going a million times faster than sound. However electronic equipment can measure the effect on light and other electromagnetic waves. For example a police radar gun uses the effect to measure the speed of vehicles.
Christian Doppler
The first person to explain this effect was the Austrian maths teacher Christian Doppler in 1842. He followed up his idea with experiments involving putting musicians on open railway carts. With another musician on the station platform the musicians could compare notes played to notes heard. Doppler showed that the effect was the same regardless of whether the note was played on the moving train of the platform. It was all to do with relative movement.
Doppler thought that the reason that double stars were often different colours was due to his effect. He was wrong! There would have been a slight colour shift as the stars orbited around each other but not one he could have detected at the time. However, measuring the Doppler shift of light is a vital tool used by astronomers today. With it we can find out the dynamics of the universe, from the speed of a Sun flare to the rotation of an entire galaxy and the expansion of space itself!
One of the most famous astronomical photographs, the Hubble deep field shot of galaxies in Ursa Major does show some visible reddening of the most distant galaxies. This is because the space between us and these immensely distant objects is expanding, making our relative positions rush away from each other at a significant proportion of the speed of light. This stretches out the waves we see from the galaxies, making them appear redder.
How do you know that an object like a star is bluer or redder because of its movement and not because it's really a red or blue star? Have a look at the 'Fingerprints of Light' page. Different elements mark the spectrum of a star at precise wavelengths. If the spectrum is reddened or blued these lines get shifted out of their precise positions. If the object is coming towards us the lines are blue shifted. If the object is going away they are red shifted.
Astronomers are able to measure tiny blue and red shifts using these marker lines. The specra below show the redshift of light coming from the stars in a supercluster of galaxies 1 billion light years away. Because of the expansion of the Universe, they are moving at a speed of 7% of the speed of light relative to us. you can easily see the shift in the yellow sodium lines which have been moved towards the orange part of the spectrum.
