# Doppler effect star and earth question

1. Oct 5, 2006

### valueduser

Hello I have a question that is giving me trouble:

Light of wavelength 659.7 nm is emitted by a star. The wavelength of this light as measured on Earth is 661.1 nm.

How fast is the star moving with respect to Earth? km/s

Is it moving toward Earth or away from it?

Here's where I'm at. I have reasoned that since the wavelength is increasing that the star is moving away. My main problem is determining the frequency of the light. I assume that I can use the formula:

velocity = wavelength * frequency

however the frequency I get just seems wrong. The units are throwing me off too, any help would be greatly appreciated.

2. Oct 5, 2006

### Staff: Mentor

What is the equation for the change in frequency of a wave based on its speed and the speed of the emitter? And keep in mind that the speed of light is about 300*10^6m/s (check this before using it, though), so that gives you a way to convert between frequency and wavelength.

EDIT -- Fixed typo in speed of light. It's 300*10^6, not *10^8. Sorry.

Last edited: Oct 5, 2006