## GRE0877 Q#55

Catria
Posts: 353
Joined: Fri Oct 26, 2012 4:14 pm

### GRE0877 Q#55

A distant galaxy isobserved to have its hydrogen-b line shifted to a wavelength of 580 nm, away from its laboratory wavelength of 434 nm. Which of the following gives the approximate velocity of recession of the distant galaxy? (Note: $\frac{580}{434}=\frac{4}{3}$)

How could the recession speed be 0.28c?

TakeruK
Posts: 838
Joined: Mon Jan 02, 2012 3:05 pm

### Re: GRE0877 Q#55

You have to apply the relativistic doppler effect (http://en.wikipedia.org/wiki/Relativist ... ler_effect)

$\frac{\lambda_o}{\lambda_s} = \sqrt{\frac{1+\beta}{1-\beta}}$

The fraction on the left is 4/3, as the question said. So you just have to solve for beta, and beta=0.28 works. (beta=v/c)

Catria
Posts: 353
Joined: Fri Oct 26, 2012 4:14 pm

### Re: GRE0877 Q#55

TakeruK wrote:You have to apply the relativistic doppler effect (http://en.wikipedia.org/wiki/Relativist ... ler_effect)

$\frac{\lambda_o}{\lambda_s} = \sqrt{\frac{1+\beta}{1-\beta}}$

The fraction on the left is 4/3, as the question said. So you just have to solve for beta, and beta=0.28 works. (beta=v/c)

Thank you... with 9677 and 0877 for practice materials I will continue studying for the April test.