It’s a good comparison because both sound and light travel in waves, which are affected by their movement. Today, astronomers use these words to describe changes in the wavelengths of radio waves or X-rays as comfortably as they use them to describe changes in visible light. So fast in fact that it is relatively easy to measure the shift in their spectral lines. That change in the sound of an ambulance is due to what’s called the Doppler effect. Because the Doppler effect was first used with visible light in astronomy, the terms blueshift and redshift became well established. That's about 300 meters per second for sound, but 300 million meters per second for light! So it's easy to hear the change in pitch when a car whizzes past you, but it's very hard to detect the change in color of the car! On the other hand, a very sensitive device could measure this change, which is just what happens when a speeding motorist is caught in a radar speed trap.ĭistant galaxies are moving away from us extremely fast. The main difference between light and sound is that to get a detectable Doppler effect you have to be moving at a speed which isn't tiny relative to the speed of the waves. But in general it's just the same: if you're approaching a light source you see shorter wavelengths (a blue-shift), while if you're moving away you see longer wavelengths (a red-shift). In detail the amount of shift depends a little differently on the speed, since we have to do the calculation in the context of special relativity. The same principle applies for light as well as for sound. Since long wavelength's (low frequencies) mean lower notes and shorter wavelengths (higher frequencies) mean higher notes, then you hear a higher pitch if you and the source are approaching each other and a lower pitch if you're moving apart. Similarly if you're approaching the source, then you'll be meeting each crest a little earlier, and so you'll perceive a shorter wavelength. Astronomers can learn about the elements in stars and galaxies by decoding the information in their spectral lines. If you are moving away from the source (or equivalently it is receding from you) then each crest will take a little longer to reach you, and so you'll perceive a longer wavelength. Describe how we can use the Doppler effect to deduce how astronomical objects are moving through space The last two sections introduced you to many new concepts, and we hope that through those, you have seen one major idea emerge. The explanation is that the sound waves have a fixed wavelength (distance between two crests or two troughs) only if you're not moving relative to the source of the sound. When it is approaching you the pitch is high, and it gets lower as it passes you and recedes into the distance. Everyone is familiar with the sound of a passing car.

It is simplest to think of Doppler shift for sound waves.

Doppler shift astronomy