Describe selection rule and intensities
selection rule describes how the probability of transitioning from one level to another cannot be zero. It has two sub-pieces: a gross selection rule and a specific selection rule. A gross selection rule illustrates characteristic requirements for atoms or molecules to display a spectrum of a given kind, such as an IR spectroscopy or a microwave spectroscopy.
Intensity is the magnitude of a quantity (such as force or energy) per unit (as of area, charge, mass, or time