Optics B/C

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Re: Optics B/C

Post by cngu23 » February 28th, 2012, 4:44 pm

JSGandora wrote:If it's moving away, then the wavelength will increase therefore it's a red shift. If it's moving closer, then the wavelength will decrease meaning it will be a blue shift.
For calculation, what is the unit measure of z in the redshift formula?
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Re: Optics B/C

Post by AlphaTauri » February 28th, 2012, 4:46 pm

cngu23 wrote:
JSGandora wrote:If it's moving away, then the wavelength will increase therefore it's a red shift. If it's moving closer, then the wavelength will decrease meaning it will be a blue shift.
For calculation, what is the unit measure of z in the redshift formula?
z has no unit - it's just a dimensionless number.
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Re: Optics B/C

Post by JSGandora » February 28th, 2012, 4:47 pm

Instead of using that, which doesn't help at all in this case, use the Relativistic Doppler Shift equation which is this:
http://hyperphysics.phy-astr.gsu.edu/hb ... ldop2.html

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Re: Optics B/C

Post by cngu23 » February 28th, 2012, 4:58 pm

JSGandora wrote:Instead of using that, which doesn't help at all in this case, use the Relativistic Doppler Shift equation which is this:
http://hyperphysics.phy-astr.gsu.edu/hb ... ldop2.html
So if I use this equation instead of the one of doppler effect, for my previous problem, what would I use for v?

And there may be an error in the answer key for my previous problem. If an object is traveling away from the observer, I believe that the observed frequency would be less than the actually frequency, but the answer says that it increases by a factor of 10.
Can someone verify this?

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Re: Optics B/C

Post by JSGandora » February 28th, 2012, 5:03 pm

Yes, the frequency should decrease. I think I took that test before and my answer disagreed with the answer key.

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Re: Optics B/C

Post by Infinity Flat » February 28th, 2012, 6:01 pm

JSGandora wrote:Yes, the frequency should decrease. I think I took that test before and my answer disagreed with the answer key.
My mistake, I used the Doppler Effect calculator linked above, and I may or may not have solved in the wrong direction. :P
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Re: Optics B/C

Post by cngu23 » February 28th, 2012, 7:08 pm

JSGandora wrote:Yes, the frequency should decrease. I think I took that test before and my answer disagreed with the answer key.
Alright. So should the correct answer be 2.56 x 10^6? I used the doppler effect equation.
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Re: Optics B/C

Post by Infinity Flat » February 28th, 2012, 7:23 pm

cngu23 wrote:
JSGandora wrote:Yes, the frequency should decrease. I think I took that test before and my answer disagreed with the answer key.
Alright. So should the correct answer be 2.56 x 10^6? I used the doppler effect equation.
Using this calculator for relativistic Doppler effect, I get 8.00 * 10^5 Hz.
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Re: Optics B/C

Post by JSGandora » February 28th, 2012, 7:27 pm

Sooo confusing @.@

So plugging the numbers in and using Wolfram Alpha, I get 1.25*10^6:

http://www.wolframalpha.com/input/?i=5s ... %29*10%5E6

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Re: Optics B/C

Post by cngu23 » February 28th, 2012, 7:35 pm

JSGandora wrote:Sooo confusing @.@

So plugging the numbers in and using Wolfram Alpha, I get 1.25*10^6:

http://www.wolframalpha.com/input/?i=5s ... %29*10%5E6
My bad. I was using the wrong doppler effect equation. The one for sound.

for your wolframalpha calculation, you used 2.6461 instead of 2.8481. And also, v should be the velocity of the wave, not frequency. But since they are directly related, should that not matter?

EDIT: Actually, I used a calculator to verify your wolfram alpha calculation with 2.8481 instead of 2.6461. It turned out to be 8 x 10^5
Last edited by cngu23 on February 28th, 2012, 7:40 pm, edited 2 times in total.
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