What is the Doppler effect? The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
All energy travels in waves. Some waves, such as sound waves, have to have a medium to travel through and some waves, such as light waves, don’t need a medium to travel through. Sound travels because the source of the sound makes the closest atoms vibrate and those atoms pass the vibrations on to the next, and so on. Sound waves constantly lose energy as they travel, which is why they die out quickly and which is why they travel further in water than in air. Light waves don’t need a medium to travel through. They are a wave of pure energy, and they don’t lose any of that energy unless they hit something.
Any kind of a wave has a frequency. The frequency of a wave is the number of waves that pass a fixed place in a given amount of time. The higher the frequency, the more energy the wave has. For example, the lowest sound humans can hear is 20 Hz. That means a sound wave with a frequency of 20 waves in one second. The highest sound we can hear is 20,000 Hz, which is 20,000 waves in one second. The lowest frequency of light we can see is 400 terahertz, which is 400,000,000,000,000 waves a second. Incidentally, we see that as violet.
So, what is the Doppler effect? Picture a lake that is 100 m across. My friends Joe and Alex stand on opposite edges of the lake, and I am in the middle in a rowing boat. I have a huge pile of stones in the boat, and, for whatever reason, I drop one of those stones in the water every five seconds. The ripples will radiate out from the boat and reach Alex and Joe at exactly the same time and at a frequency of 1 ripple every five seconds. Now, if I keep dropping the stones in the lake, but I start to row towards Joe at the same time, the frequency for him is going to change. As I get closer to Joe, the ripples take less time to reach him because the point of origin is getting closer to him. The frequency of the ripples I am creating hasn’t changed, but the frequency of ripples for him goes up because the ripples are bunching together. However, I am getting further away from Alex, so the ripples take longer to reach him. Again, the frequency of the ripples I am creating hasn’t changed, but the ripples take longer to reach him, so the frequency for him goes down. This is the Doppler effect.
The famous example of this is the ambulance with its siren going. The sound from the siren is a sound wave and it has a specific pitch. As the ambulance is approaching you, it is getting rapidly closer, so the soundwaves don’t have as far to travel and they bunch up, making the frequency is higher. Frequency equals pitch with sound, so the sound of the siren is higher pitched. Once the ambulance has passed by, the source of the sound is getting further away and the frequency of the waves drop because they take longer to reach you, giving a lower pitched sound.
The Doppler effect is named after Christian Andreas Doppler who was an Austrian mathematician and physicist. He came up with his idea that became known as the Doppler effect in relation to stars and not to ambulances. Some stars appear different colors and Doppler thought this could be explained by the speed of the star relative to us.
Astronomers use this method to estimate how far away stars are and to know if they are coming towards us or moving away from us. Stars release light which has a specific wavelength. If the star is moving towards us, the waves bunch together and the frequency for us increases. Light with a high frequency is blue, or violet. If the star is moving away from us, the waves spread out and the frequency for us drops. A light wave with a low frequency is red. So, if the star appears blue, we know it is coming towards us, and if it appears red, we know it is moving away from us. This is redshift and blueshift. Using this information, Edwin Hubble was able to show that the universe is expanding.
So, what is the Doppler effect? It is the effect when the source of a wave is moving towards or away from the observer. If they are moving towards the observer, the frequency increases and if they are moving away from the observer the frequency decreases. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Doppler_effect
https://en.wikipedia.org/wiki/Christian_Doppler
https://economictimes.indiatimes.com/definition/doppler-effect
https://www.uspto.gov/kids/MiddleSchool-HowWellSoundTravels.pdf
https://www.qrg.northwestern.edu/projects/vss/docs/communications/1-what-is-frequency.html
https://www.lumitex.com/blog/visible-light-spectrum
https://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect