The site http://physics.nist.gov/PhysRefData/Handbook/Tables/hydrogentable3.htm shows the intensity of various photon energy values from hydrogen. I was not able to see how this intensity was observed (ie. was it from a specific experiment, or does one simply look at the sun and count photons at this energy?). The references, from what I could see, seem to deal more with calculation of levels rather then calculating observed intensity. To make the long question short: Why is the 2p3/2 to 1s1/2 (1215.66824 angstroms) transition twice the intensity of the 2p1/2 to 1s1/2 (1215.67364 angstroms) transition? Is it because the 3/2 has 4 states and the 1/2 only 2 states and we assume all the states are equally populated? Another question: Why is the 2p to 1s transitions 5 times more likely then the 3p to 1s transition? ... what is the general rule for understanding the intensity column?