Worked example - Zeeman splitting in a 1-tesla field (normal effect). The shift is delta-E = mu_B x B. Step 1: mu_B = 9.27 x 10^-24 J/T, B = 1 T, so delta-E = 9.27 x 10^-24 J. Step 2: as frequency, delta-nu = delta-E / h = 9.27 x 10^-24 / 6.626 x 10^-34 = 1.4 x 10^10 Hz = 14 GHz. Step 3: as wavelength near 500 nm, delta-lambda = lambda^2 x delta-nu / c = (5 x 10^-7)^2 x 1.4 x 10^10 / 3 x 10^8 = 1.17 x 10^-11 m = 0.0117 nm. Small but resolvable - and in sunspots (B ~ 0.3 T) the splitting is routinely measured to map solar magnetism.