def rydberg(n1,n2) :
    """ Rydberg formula to give H wavelengths for transitions between levels n1 and n2
    """
    r=1.0973731568e7
    me=9.109382e-31
    mprot=1.672621e-27
    rm=r/(1+me/mprot)

    l=rm*(1./n1**2-1./n2**2)
    w=1./l*1.e10
    return w