Imagine that you are an ambulance driver and you have to drive at high speed along the streets of a big city filled with cars. Now imagine that you are one of the crowd on the sidewalk. You are standing at the crossing and waiting for the moment when you can cross the street. But first you need to skip the racing ambulance.
The roar of her siren is heard from afar. But the strange thing is, the closer a car with a red cross drives up, the higher the sound of a siren becomes. When the car starts to move away, the same thing is repeated, but vice versa. As the car moves away, the sound of the siren becomes lower and lower until it completely disappears. At the same time, the ambulance driver does not notice any changes. For him, the sound quality does not change.
But an outside observer hears how the pitch rises and how then the tonality decreases with distance. Sound waves propagate in the air in the same way as sea waves on the surface of the water.
So what really happens. Who is hearing right? A driver or a pedestrian? Does the tone of the siren sound change? Both are right. More precisely, no one is mistaken: both the driver and the pedestrian hear exactly what they should hear. The difference in perception is due to the Doppler effect. What we hear as sound is actually waves propagating through the air.
The siren makes air molecules vibrate. Sound waves propagate in the air in the same way as sea waves on the surface of the water.A wave is a region of rarefaction, which then becomes a region of compression. The process is repeated many times in one second and spreads. This is the sound wave. The closer the same sections of the waves are to each other, the higher the sound, that is, the greater its frequency.
In our case, when the “fast” wave approaches, the sound waves become closer to each other for the pedestrian, because the speed of movement of the car and sound add up. The smaller the distance between the sound waves, the higher the frequency and the higher the sound tone. With the removal of the machine, the distance between the waves with increasing distance becomes more and more, that is, the frequency decreases gradually and the sound becomes lower. The people in the car and the sound source are motionless relative to each other. Therefore, no changes in tonality occur. In order to hear changes in tonality, the listener and the sound source must move relative to each other.
Doppler effect not only in sound waves
Take light waves as an example. If a yellow lamp were installed instead of a siren on an ambulance, then when approaching the observer, the spectrum of the lamp would shift to the blue side, and when removed, to the red one. With the usual phenomena surrounding us, the displacement rates are relatively low, so we do not notice changes in the light spectrum. But if the speed of the ambulance was approaching the speed of light or comparable with it, then we would notice the desired changes.
Frequency is the number of wave crests that have passed through a specific point in one second. The higher the frequency, the higher the tone of the sound or the more blue the light becomes.The driver in this case would see a yellow light constantly falling on the road. But a moving machine would compress the waves in front of it and observers who were motionless while approaching a light source would see a shift of the light spectrum towards the high-frequency blue side. As the car moves away, the observer would notice the color of the flashlight return from blue to yellow. Gradually, this color would turn into red, disappearing over the horizon.
