the Doppler effect.


I know a car sounds different approaching vs leaving but what is happening re sound waves and is there a similar effect with light waves?

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When moving towards you, the speed of the thing making the sound is squishing the wave due to its speed.

As it moves away, it’s speed is stretching the wave out.

The length of the wave determines how it sounds

When the source of is approaching you, the sound waves get sort of “compressed” giving off a higher frequency/pitch. Then when that source passes you and is now traveling away, the sound waves get stretched out having the opposite effect, giving off a lower frequency/pitch.

The sound waves are squished because as a vehicle approaches you the individual impulses need to travel a shorter distance each time making them reach you faster. The opposite happens when a vehicle moves away from you.

And yes, this works for light too. Iirc we use it to determine how fast objects in space are moving relative to Earth.

Say you’re standing on the ocean shore and the waves are splashing on your feet at a steady interval. If you run out into the ocean you will get hit with more waves than standing still over the same amount of time. Now run back out of the water. You probably aren’t fast enough to outrun all the waves but you’ll be hit by fewer than when standing still.

When it comes to sound the energy in the noise is constant, like a 3ft string will always be 3ft long but you can make a wave that is 3″ long but the top and bottom would have to be pretty high compared to the string laid flat or with a slight wave.

So the ambulance moving closer while using the siren means that more waves hit you faster, the waves have to change shape and get taller to keep the same energy. Taller waves sound higher. Then the reverse happens when it’s moving away. The energy always stays the same it just changes shape to fit in the space available

You’re literally running into/away from sound waves, causing you to receive more or less waves so they sound differently.

The same phenomenon is also present on lights but for very different reasons. Everyone approaches light at the same speeds, but the definition of space and time changes, making the light to appear to change in frequency.