Photon

The main property of a photon is its energy, which is linked to its frequency through Planck's constant $h$ as $E=h\nu$ . This relates its energy to its wavelength numerically as:

$E{\text{ (eV)}}={\frac {1.2398}{\lambda {\text{ (μm)}}}}$

Light waves have wavelengths between about 400nm (violet) and 700nm (red), which makes the energy of visible light ranges from 1.7eV to 3eV (red is less energetic than violet; infrared is innocuous, ultraviolet is dangerous).

In Hydrogen, the Balmer series is the visible one: it involves transitions from all excited states down to $n=2$ (2nd excited state). Transition to $n=1$ are more energetic (in the UV) and correspond to the Lyman series. Using Bohr's formula:

$\Delta E\text{ (eV)}=13.6\left[{1\over n_f}-{1\over n_i}\right]$

one can check that that the $n=2$ to $n=1$ Lyman photon has energy 10.2eV and is thus not visible. Splitting it in two, however, makes the photon with smallest energy fall in the visible range, providing a continuous visible spectrum (since only the sum of the energies is quantized).

We are interested in both single-photons as well as multiphotons.