Light waves

1. Oct 12, 2006

disregardthat

I cannot search the forum to check if this has been posted before, because I do not know what it is called in english...

That means that i have to try to explain what i means, here it goes: When a light producer, like a star is moving, it sends out waves that is either compressed or expanded. You know what i mean?

Anyway, if the star was moving half the speed of light, will the waves be compressed to a half? Example: If the sun sends out waves at a wavelength of 800 nm, and moves TOWARDS us in the speed of light\2 (a half), would the wavelenght be 400 nm?

And let's say that the star stands still, but the earth is moving half the light speed, would we observe the waves as half?

2. Oct 12, 2006

Hootenanny

Staff Emeritus
One such search for the Doppler Effect [For light].

3. Oct 12, 2006

Staff: Mentor

And if the source (or the observer) is moving as fast as half the speed of light, you should use the relativistic Doppler effect equation, not the non-relativistic one that people usually learn first.

4. Dec 26, 2006

disregardthat

What is the difference between the relativistic doppler effect and the non-relativistic doppler effect? and what's the equation? (I usually don't udnerstand all the units so the hard ones can yuo please define? would be great)

Trying to put my teacher in his place although physics isnt his main subject.

5. Dec 26, 2006

Staff: Mentor

The equations for both Doppler effects are compared here: Relativistic Doppler Shift

Perhaps this isn't the best use of your time.

6. Dec 26, 2006

disregardthat

Yeah it is! Not only to put him in place, but to understand it myself!

Thanks by the way

7. Dec 26, 2006

disregardthat

Ok i used the formula of Relativistic Doppler Shift, with the velocity between the observer and source to be 100 000 000 m\s. It came with this answer:

141458078,17020046914680279765556

Maybe I have done something wrong. The V divided by c in each bracket is supposed to be the same as Vsource right? If not, what should it be?

8. Dec 26, 2006

Staff: Mentor

No idea what you are calculating. The V in V/C is the relative speed of observer and light source. The $\nu_{source}$ is the frequency of the source (that's a "nu", not a "v").

That site has a calculator built in: Doppler Calculation

9. Dec 26, 2006

disregardthat

And what excactly does the frequenzy of the source means? The frequenze of the emitted light waves?

All right, thanks, I could use the calculator, but i have 1 question. for the equation, is the frequenzy supposed to be like this for example: 3*10^14?

Last edited: Dec 26, 2006
10. Dec 27, 2006

Hootenanny

Staff Emeritus
Yes, the source frequency is the frequency at which the light is emitted from a source moving relative to an observer.

11. Dec 27, 2006

disregardthat

What is the logarithm in the electromagnetic specter? Do any of you have an equation for the graph?

Last edited: Dec 27, 2006
12. Dec 27, 2006

Staff: Mentor

Not sure what you are doing. What makes you think you are getting redshifts?

13. Dec 27, 2006

Hootenanny

Staff Emeritus
Now I'm confused... perhaps if you could explain your question further, or post your working on the problem you are attempting.

14. Dec 27, 2006

disregardthat

Uhm, sorry about the redshift comment, a little silly mistake by me, I edited my post. The reason I thought I was getting redshifts, was because the frequency of the wavelengths i got was lower when the source was moving TOWARDS the observer. My mistake was that turned the bracket inside the equation... Not so experienced with physic equations yet, trying to learn a bit.

Ok, about the logarithm question. Logarithmic graph IS when the line doesn't go straight, but in an accelerating rate for example upwards, right?

Like y=2^x is a graph where the number 2 doubles for each point on the X line. Like this:

X: 2, 3, 4, 5, 6, 7
Y: 2, 4, 8, 16, 32, 64

I'm not sure the definition, but in that graph the acceleration doubles on each point.

So, I was wondering what the logarithm was in the electromagnetic specter. Since it kind of looks like the PH meter.
The electrodynamic specter have 3*10^14 at one point, 3*10^15 at another, and 3*10^16 at another. And the length between the points are equal, but the number are not at all! So I wondered what graph you could use to show thsi logarithm, because it's not very lucid<-(is that the word? at least, not a very good overview to see the difference between the low and the high wavelentgths\frequencies)

Last edited: Dec 27, 2006