Do I have enough information to solve this problem? If so....how do I solve it???

Question

A star approaching us at 300 km/s will show the H alpha spectral line a) redshifted by 0.66 nm b)blueshifted by 0.66 nm c) redshifted by 6.56 nm d) blueshifted by 6.65 nm

Answer 1

Yes,you should have enough information.  The shift in the Halpha  line is = Halpha wavelength x 300 km/s / speed of light.  Since the star is approaching, the shift will be a blue shift.

Answer 2

You need to know the wavelength of the H alpha spectral line and the speed of light.

Then you just plug them into the Doppler shift equation for speeds much slower than the speed of light.

Answer 3

f' / f = ( c / c + 300 km /s )

The H alpha line has frequency 656.3 nm.

300 km / s is about .0001 c

So the spectral line will be blueshifted by about .66 nm.

Answer 4

Yes you have enough information.

You need to find the "lab" line (wavelength) of H alpha (also called the wavelength "at rest"), and find by how much the line is shifted at that speed.

However, given the choices, it will be easy to figure out the answer once you have the base wavelength (656.28 nm times 300/300,000)

two answers can be eliminated (approaching = blue shift) leaving you with one that is correct and one that is off by a factor of 10.