One of the most terrific things about science is how two seemingly unrelated things can connect together. You come to realise that everything is just a thick interdependent web where a topic can climb its way to grab on to another. It’s a linking chain of information, constantly building up and often solving puzzling mysteries. However, it also produces unexpected mysteries, mysteries that confound the best of us. For every answer received, dozens more questions seem to arise.
The Doppler Effect (or Doppler shift) is a prime example of this. Named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague.
A simple observation noticed here on Earth has led to great advances in astronomy. So, what is the Doppler shift? I here you ask.
Doppler shifts can happen wherever we experience waves. For example light and sound consists of waves. Doppler shifts are more common to us through sound or by water and I assure you, you’ve all probably experienced it before (unless your a hermit and in that case I have no idea how your reading this post). When you are standing by the roadside watching the cars whizz by at high speed, the sound of the car’s engine seems to drop in pitch as it passes you. You know that the car’s engine note stays the same, so why does the pitch seem to drop? It’s because of the Doppler shift.
Sound travels in waves of changing air pressure. When you listen to anything, what you’re actually hearing is the changes in air pressure. Sound created by the car’s engine travel through the air in all directions from the source. If your close enough, your ear happens to lie in one of these directions, it picks up the changes in air pressure and your brain translates it as sound. Don’t imagine particles of sound flying through the air to your ear. It doesn’t work like that. Sound waves travel outwards in all directions, like the ripple created if you drop something in a body of water. The ripple you see when dropping something on water is just an example of the Doppler effect occuring. But this time it’s in water rather than the air.
The best way to understand it is by thinking about the Mexican wave. The impressive effect audiences do in concerts or in packed sports stadiums.
As you’ve probably noticed each person stands up then sits down again, immediately after the person on their side has done so. A wave of standing and sitting moves around the stadium. Nobody actually moves their place because they all sit back on the same seat. Yet, the wave travels. It travels quite fast as well.
The ripple effect is like the Mexican wave. What travels in the pond is a wave of changing height in the surface of the water. The water molecules themselves are not moving away from the thing that dropped (let’s say a small rock) they’re just simply going up and down. Just like the people in the stadium.
The Doppler effect is slightly different in sound. As I’ve mentioned before, what travels in the case of sound is a wave of changing air pressure. The air molecules around the source of the sound wriggle to and fro. As they do so, they wriggle the air molecules around them. This causes a chain reaction of wriggling air molecules until they reach someone’s ear. The vibrating air molecules are then translated into sound by the brain. Keep in mind that the air molecules closest to the sound source do not travel to your ear, It is the wave that travels. The wave always travels at a fixed speed, regardless of what the source of the sound is, It could be a guitar or a car’s engine or a pair of headphones.
Sound travels 768 miles per hour in air, four times faster in water and even faster in some solids.
The sound wave travels at a fixed speed even if the pitch of the note is altered. The sound wave produced when a high note is played on a guitar travels at the same speed as a low note. However, the distance between the wave-crests (the wavelength) is smaller on a high pitched note than on a low pitched note.
So now we know how sound waves travel, what about the Doppler shift?
Imagine your’re by the road again but this time you’re in a car driving along. There is someone by the middle of the road blowing a loud horn. What will you hear? The successive wave crests leaving the horn at a definite distance from each other, defined by the note the horn is blowing at.
The diagram above shows what the sound waves would be like if you were standing stationary near the horn. But, as your car is whizzing towards the horn, your ears will pick up the successive wave crests at a higher rate. This way, the note the horn is playing at will seem higher than it really is. After, once your a long distance away from the horn-blower, the sound waves-with their wave crests- will hit your ear at a lower rate. They’ll seem more spaced out because each wave crest is travelling in all directions. It will seem more lower pitched than it actually is. For this example I’ve shown the sound source as being stationary while you are moving. Most examples depict the sound source moving while you are you are stationary. It works both ways.
I’ve now explained the Doppler effect acting on sound and on water. In part 2 I will be explaining it’s relevance in astronomy and radar and it’s affect on light. Stay tuned folks 🙂