# Help! Lyman line / relativity

1. Dec 18, 2004

### yxgao

Hi,
What is the effect of relativity on the Lyman line? For instance, an object emits a line with a certain wavelength, but we measure it to be 607.5 nm on earth. What is the speed that the object is moving relativity to the earth?

Thanks.

2. Dec 18, 2004

### dextercioby

I believe it has to do with the old (round 150 years) Doppler-Fizeau effect.Take the book,search for the formula and apply it correctly,knowing u're given the value in the moving (but inertial) reference frame and you're being questioned about the wavelength in the rest reference system.

Daniel.

PS.It be can't gravitational red shift,right?? :surprised

EDIT:You're questioned about the velocity,you're given both wavelengths.So it's the same formula,though...

Last edited: Dec 18, 2004
3. Dec 18, 2004

### Staff: Mentor

Doppler effect - as an object moves away, the frequency of light emitted is reduced (red shift), and conversely, as the object approaches, the light wave frequency would increase (blue shift).

4. Dec 18, 2004

### Gamma

Frequency as observed from earth = sqrt[(c-v)/(c+v)] * frequency of the source.
v>0 means the object is moving away from the observer (red shift).

gamma.

5. Dec 18, 2004

### yxgao

$$\nu_{obs} = \nu_{source} \sqrt{{\frac{1+\frac{v}{c}}{1-\frac{v}{c}}}}$$
$$\nu$$ positive means source is approaching
$$\nu$$ positive means source is not approaching