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Doppler effect with light

FireMyLaser

FireMyLaser

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I found some wiki-stuff; http://en.wikipedia.org/wiki/Doppler_effect

So I understand it like this: (?)
An infrared light source is traveling at the speed of light or very fast towards the observer. As the light source gets closer to the observer (eye), the lightwaves get compressed, therefor shorter, just like soundwaves can. The resolut would then be that the light will change from IR to red, then to orange, and so on untill it reaches gamma-rays, maybe.
Get it?
 





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nikokapo

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Think about it like this:

*sorry im too busy to explain everything right now, but i'll come back tomorrow or later, but i'll leave you this quick mspaint drawing :p*

if you start from this image, you can tell that this is why you hear that SWOOOOSH sound a bit later (disregarding distance to the plane) and as the plane goes farther away the frequency of the sound varies



EDIT: This is better

doppler_w_vel.gif





EDIT 2: this is even better:

doppler.gif
 

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FireMyLaser

FireMyLaser

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Yes-yes, very good, but now it's all about LIGHT. We all have experienced the doppler effect with sound anyway. ::)

Are my thoughts correct, and is it possible?
 
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nikokapo

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Oh....yeah sorry.

Erm, i kinda agree with what you said

i always imagined a "visible spectrum (plus IR and UV)" rectangular bar
and when applied the doppler effect on an observer on the center or one of the two ends of the bar, the spectrum would move towards or away from the observer & thus "moving" light to another wavelength.

but as you can see from the first and second imgs. i've linked, the wavelength gets indeed compressed.
 
diachi

diachi

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This probably wouldnt happen with light, as light isnt the same as sound, yes they are both energies. Light will always move away from or towards you at roughly 300,000,000 m/s so there is no compression of the waves, where as the speed sound chanes depending on the speed of the vehicle causing the waves to compress.

If you understand that, you may not, I will find a better way to describe it later.

FrothyChimp will most likely have an easier to understand explanation than me.

Diachi
 
FireMyLaser

FireMyLaser

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Sorry, I don't undestand. :-[

... maybe you mean that nothing can travel at the speed of light besides light, but let's forget about that.

Infact, how about this one, just for fun:
What if you had a very bright (let's forget about divergence) laser traveling at the speed of light and fire the beam backwards. Would the end of the beam just stop dead?
I'm not very smart, but I like to think.
 
diachi

diachi

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well some scientists did something that made light have a negative speed because they made it travel so fast...

Diachi
 
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nikokapo

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FireMyLaser said:
Sorry, I don't undestand. :-[

... maybe you mean that nothing can travel at the speed of light besides light, but let's forget about that.

Infact, how about this one, just for fun:
What if you had a very bright (let's forget about divergence) laser traveling at the speed of light and fire the beam backwards. Would the end of the beam just stop dead?
I'm not very smart, but I like to think.
Click to expand...

Mein answer:

A. the beam would reflect back and hit the diode, killing it instantly or slowly.

B. if it reflects, why would it stop? lets say it reflected in a 30° angle. then it'll continue its way :S
 
diachi

diachi

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but you can actually make it travel back without it reflecting, by giving it negative speed.

Diachi
 
FireMyLaser

FireMyLaser

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What are you guys talking about? "Negative speed, 30° angles". I'm a fysics n00b.

I suck at explaining things, I really tryed to explain this to one of the teathers but he didn't get it after 5 minutes so I gave up.

What I mean is that they cancel each other out. The laser itself goes that ---> way at speed of light with the tailcap first, and fires beam so it goes that <--- way.

300,000,000 - 300,000,000 = 0 m/s, that's the speed of light in my world. But what's the catch? ::)


Anyway it's kinda OT, just one of the things inside my head. I would like to know more about the doppler effect.
 
MarioMaster

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of course this effect works with light - it's used a lot in space and astronomical kinds of stuff, usually under the names of "red shift" and "blue shift" and works exactly as you described
 
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likewhat

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one interesting example of the doppler effect that someone eluded to is when you have an ultrarelativisitic particle in space the cosmic microwave background gets blue shifted up to extremely hard x-ray frequencies in the rest frame of the traveling particle so they are energetic enough for pair production when they collide with the particle.
 
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nikokapo

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Diachi said:
but you can actually make it travel back without it reflecting, by giving it negative speed.

Diachi
Click to expand...


that'd be negative acceleration. but i think fire said he didnt want any negative thing :p
 
pullbangdead

pullbangdead

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Doppler effect with light waves happens, and gets used in the real world everyday all across the world. Police use radar guns to measure speed of moving objects, baseball scouts use radar guns to measure the speed of pitches, people use radar guns to measure the speeds of things all the time. Radar guns use radio waves and the Doppler effect, measuring the shift of the radio waves after they bounce off of objects to calculate the speed. This is radio waves, but it's still exactly the same thing as visible light, just a longer wavelength.
 
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