calculates the speed at which a star is moving away/towards us with the help of redshift/blueshift obtained from radiations emmitted from the star. at 656.28nm there should be a sharp decrease in the emmitted radiation since hydrogen(present in stars) absorbs all of those radiations onto itself. From the graph we will check the difference in the minimum value with 656.28 and find the red shift or blueshift and hence determine the stars' speed.