(adsbygoogle = window.adsbygoogle || []).push({}); 1. Astronomers on Earth observe a feature at 442 nm in the spectrum of a distant quasar. However, it is known that this feature corresponds to 581 nm in the rest frame of the quasar. Calculate the speed of the quasar. Enter your answer as a fraction of the speed of light, positive if the quasar is approaching, negative if it's receding.

2. Relevant equations

I used

3. The attempt at a solution

It was easier for me to think about this in terms of frequency than wavelength, so I converted the wavelengths into frequency by using

c = speed of light

L = lambda, wavelength

f = frequency

u = speed of source

c/L = f

So 3e8/442e-9 = 6.79e14

3e8/581e-9 = 5.16e14

So the observed frequency is 6.79e14 Hz and the actual frequency is 5.16e14 Hz.

f_{obs}=(1-u/c)f_{emitted}

6.79e14 = (1-u/c)(5.16e14)

1.316 = 1-u/c

.316=-u/c

-.316c=u

Now, this isn't right. I'm using (1-u/c) versus (1+u/c) because the relative speed of the object is negative if they're approaching, and the wavelength is getting blueshifted, so they're moving towards each other. However, that produces a negative answer, and the question states to enter a positive answer if they're moving towards each other.

Anyone see any mistakes in my work?

Thanks a lot in advance~

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# Homework Help: Find speed of quasar from observed and actual wavelength readings

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