DOPLER EFFECT

[Jeanway 0]

NIK? ANTHING YOU WANT TO SAY?

[nik 0]

Consider this. Think of a boat in the middle of the water bobbing up and down slightly so that it produces ripples. You're standing on shore with a stopwatch and you

measure the time between wavefronts as they hit the shore. Now imagine that the

boat is moving toward you, (but still making ripples at the same rate, as far as the

boat is concerned). Well, as it turns out, the ripples are closer together when they reach you. Now think of a third scenario. The boat is stationary, but you are moving

towards the boat. You encounter the ripples sooner. In either of these cases, the

wavefronts seem "compressed" or at a higher frequency/shorter wavelength.

Moving objects relative to each other will emit "waves" at different wavelengths.

 

Here's a good question:

What will happen if the boat moves away from you? You from the boat? (I will not answer this.)

 

This is called the Doppler effect, and it applies to nearly any moving object that emits waves of some sort - light, sound, wakes, etc.

 

Now, talking about my job again. One of my projects involved superluminal jets from active galactic nuclei. That is, jets of hadronic matter that apparently were emitted very fast from the cores of quasars. How did we know they were moving fast? Well, the light from them was Doppler-shifted. In fact, we had to apply relativiistic corrections to these jets. It's amazing that the light emitted may have been visible light (or whatever) but was radio waves when it reached us, or X-rays.

 

Another cool thing is that the universe is expanding. The farther out we look, the faster things are moving away. (Think about how this can be in a uniformly expanding universe. Or... think about how this can be in a uniformly rising loaf of raisin bread. They are analogous.) We can get a good estimate as to how far something is away by how much its light is Doppler shifted.

 

Jeanway, I bet you can think of about dozen more questions about this.

 

Other applications of the Doppler shift:

 

A train coming and going.

 

Police cars moving towards and away from you. This one is fascinating because a police siren is a superposition of at least two waves, and both are shifted.

[Jeanway 0]

I visualize it like a big cone, flipped over does that make any sense to you nik? Coming at you the tiny point, going away the large base, small end or point, higher frequency, base lower frequency.

[Q stole my bike 0]

Next time you're driving around pay attention to how the engine of cars that go by sound as they pass. They sound higher as they come towards you, and lower as they go away (it sounds like nrrrrrrrrrrrroooooooooooom) That's because of the doppler effect