Spectra from Dipole Moments
The transition energy, or wavelength if you prefer, are governed by the nature of the molecules involved. Whole body free rotational motion (in the gas phase) gives rise to microwave spectra ( fractions of wavenumbers cm$^{-1}$  or THz frequencies). The frequency is determined by the molecules moment of inertia. At higher energy comes vibrational motion, stretching and bending of chemical bonds, a few hundred to a few thousand cm$^{-1}$. These bond frequencies are approximately given by $\sqrt{k/\mu}$ where $k$ is force constant which relates force to bond extension as in Hook's law, and $\mu$ is the reduced mass. There are 3N-6 types of vibration for N atoms, (3N-5 for linear molecules). At higher frequencies are electronic transitions where and electron gets promoted from a bonding to anti-bonding orbital. Typically these transitions are in the visible and uv regions.

The exact number of transitions is difficult to estimate because each vibrational level has rotational levels superimposed on them and there are also vibrations that are combinations of levels. There are, however, fewer electronic transitions because there are generally few molecular orbitals than types of vibrational modes, and each excited state has its own set of vibrational and rotational levels.

One of a chemist's essential tools, arguably more important that any other spectroscopy, is NMR spectroscopy which is caused by transitions between nuclear spin states when in a magnetic field and occurs at very low frequency, 100-800 MHz range.