Today I'm going to be explaining the Doppler effect. The Doppler effect is what happens when the sound of a fire engine seems to change as it passes you.
Now, common sense tells us that the siren's frequency didn't really change when it passed us. We've heard it happen so many times and it always happens the moment it passes us. So what's really happening? Why does it sound different?
Sound travels in waves. When something emitting a sound is moving forward the waves are compressed in front of it, and spread out behind it. The faster it moves the more compressed they are and the more obvious the Doppler effect is to our ears. Here's another example, this time with sound waves drawn in to help visualize the effect.
The Doppler effect is seen in any wave phenomenon. You may notice it more with sound waves, but it is also very important to light waves. When something that emits light is moving very fast (more exactly, moving fast compared to your frame of reference) the light waves are compressed in front of it and drawn out behind it, just like airplane example above.
This effect is very important in astronomy. It's one of the main reasons that we know the universe is expanding. Stars near the edge of the visible universe look much redder than they should (we say they are red shifted). The explanation is that the universe is expanding very rapidly and those stars are moving away from us at an appreciable fraction of the speed of light. This makes them appear red due to the Doppler effect.
| If something is moving towards you very fast it will be "blue shifted". If you were moving close to the speed of light the Doppler effect would make this red bumper sticker appear blue. |