 You have experienced the Doppler effect many times. For instance, imagine you're on the street and you hear an ambulance coming towards you. It's siren sound will be high in pitch. As it passes you, the pitch of the siren sound drops, and as it moves away from you, the pitch drops even more. The reason this apparent shift in pitch happens is because sound is a series of waves. Imagine the ambulance is stopped, and its siren is on. The waves of sound look like this:  The waves have a distinct wavelength, which is the distance between the wave crests. You can see the wavelength above, which is about 10-20 cm. The wavelength results in a specific frequency for the sound. (Frequency is another name for pitch). There will be a certain pitch associated with the siren's sound waves, which you can hear when the ambulance is not moving. Now imagine you and a friend are on the street. The ambulance starts to move away from your female friend on the left, and towards you on the right. This is what it will first look like:  As the ambulance begins to move, still emitting the siren sound at the same frequency, the waves moving towards you on the right will be compressed, while the ones moving toward your friend on the left will be lengthened.  These apparent changes in wavelength will also change how you perceive the sound. The waves you hear on the right, being shorter in wavelength, will have a higher pitch. At the same time, the waves moving left, being longer in wavelength, will have a lower pitch to your friend.  Notice that the frequency, or pitch, is inversely proportional to the wavelength. In other words, short wavelengths have high frequency, while long wavelengths have low frequency. The equation relating wavelength and frequency for sound is: frequency · wavelength = speed f · λ = vs where vs is the speed of sound, normally 343 m/s in dry air at 20 °C. It's important to note that the source of the pitch of the sound emitted by the ambulance siren doesn't change. What does change is how you perceive the sound, depending on whether the waves are moving toward or away from you. The Doppler effect is observed whenever a source of waves is moving with respect to an observer. The Doppler effect can be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. The Doppler effect can be observed for any type of wave - water waves, sound waves, and light waves. We are most familiar with the Doppler effect because of our experience with sound waves. The Doppler effect is of great importance to astronomers, who use the information about the shift in frequency of electromagnetic waves (light) received by their instruments to determine whether something is moving towards us or away from us. This could be a moon, a distant planet around another star, a star cluster, a galaxy like our own Milky Way, or the edge of a rotating neutron star. The equation is the same: frequency · wavelength = speed f · λ = cs where cs is the speed of light, normally about 300 million m/s in a vacuum. Astonomers can also use the equation to determine the speed that an object is moving toward or away from us as it emits electromagnetic waves. They also use a picture made from the light that we receive from the object, shown as a spread-out 'spectrum' of all the frequencies in that light. Here's an example. Light from a planet orbiting a star reaches us. As the planet passes in front of its star, light from the planet in that position (A), not moving towards or away from us, would, if visible, yield a 'normal' spectrum. When we look at reflected light from the right side of the planet's orbit, when it's moving towards us (B), the spectrum of light appears 'shifted' towards the blue end of the spectrum. When we look at reflected light from the left side of the planet's orbit, when it's moving away from us (C), the spectrum of light appears 'shifted' towards the red end of the spectrum. 
 In this way, using the spectrum picture, astronomers can calculate the speed of the planet in orbit. They can also determine all kinds of information about many other celectial objects. Some of this information can include the chemical makeup of stars, information about the nuclear reactions that power them, and even facts about the elements in the surface regions of the star. Red shifts have been used to measure the rotational speeds of entire galaxies of stars. Astrophysicists have used the red shift of distant galaxies to show us an amazing fact: that the farther away a galaxy is, the faster it is moving away from us. This 'red shift' is a characteristic of al far-away objects, including galaxies, clusters of galaxies, and quasars. The fact that everything seems to be moving away from us, faster and faster the further away we look, does not mean that we're at the centre of the universe. It means that the universe as a whole is expanding. The astronomer and science writer Carl Sagan described it this way. Imagine that the universe is a loaf of bread dough full of raisins. As it starts to expand, all the raisins inside are moving away from each other. The expansion would look the same from any raisin's point of view. A more familiar use for the Doppler effect is the radar gun used by police officers to record an oncoming vehicle's speed. I've used these devices in a physics class to record the speeds of hockey slapshots and pitched baseballs (long before the NHL and MLB started doing the same thing!) |