Use doppler shift to find star velocity

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Homework Help Overview

The problem involves determining the velocity of a star required to cause violet light (400 nm) to appear blue (450 nm) due to the Doppler effect. The context is within the subject area of astrophysics and wave phenomena.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to apply the Doppler shift equations but expresses confusion regarding their application and the results obtained. They present two attempts at calculating the velocity, both of which they believe are incorrect. Some participants question the clarity of the variable definitions and units in the equations used.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and the equations involved. There is a request for clarification on the use of the equations, indicating that guidance is being sought to better understand the problem setup.

Contextual Notes

The original poster notes that this problem is the last in a set of nearly 40, suggesting it may be more complex than previous problems. There is also mention of confusion regarding the template used in the thread.

Covenant32
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Homework Statement



How fast would a star have to travel to make violet light (λ = 400 nm) appear to be blue (λ = 450 nm)? Assume the star is moving along the line that connects the star to the Earth.

Homework Equations



λr = λc / (c-Vr)

f= fo (c +- Vr / c +- Vs)

The Attempt at a Solution



Well, to be honest I am completely confused with this problem. It is the last problem in a homework set, so it is most likely the most difficult.

First attempt:
400-450 = -5e-8.
Now solve for v:
Δf = fs*((1+v)/c)
v = Δf*c/fs-1 = -5e-8*3e8/(4e-7) - 1 = -3.75E7 m/s, so the star would have to be moving away from you at 37,500,000 m/s.

Second attempt:

Vr = 450 * 3e8 / 400 * 3e8 = 1.125.


Neither are correct. Any assistance or guidance is truly appreciated. Thank you.

 
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I have no idea why the template is copied multiple times. Sorry about that.
 
The answer is in terms of "c". So, I thought that perhaps I could just find the ratio of 3.75e7 to 3e8, which is .125, but that is incorrect.

I'm really lost on this blasted problem. It is the last one of close to 40 I've done.
 
I don't see how you're using the equations. Define the variables in the first equation and give the units of measure please.
 

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