Number of different wavelengths in the visible spectrum reaching Earth

I recognize the visible wavelengths of light extend from approximately 400 - 700 nm. But how many different wavelengths exist in that range? 300 ? 30,000 ? (400.01 - 699.99)

If it's completely continuous, then how can photons be so easily absorbed, as they need to match an electron potential fairly precisely to be absorbed?

(I did a naive calculation some time ago, and came up with a wavelength about every 1.5 nm, but I don't know if that is correct.)

• You are assuming that photons are only absorbed by kicking an electron to a higher energy level. But they can be absorbed by many other processes too.
– Jim
Commented Feb 18, 2015 at 15:44

To my knowledge, there is no discretization of the light wavelength (they form a continuous spectrum).

On the other hand, there exists no infinitely narrow absorption "potential". I mean that all transitions of electrons that may correspond to a photon absorption have a finite width. Consequently, the photons have a non-zero probability to get absorbed.

Moreover, many materials do not have narrow energy transitions. For example, metals have free electrons (not bound to an atom, i.e. not subject to atomic transitions) that may absorb many different photon energies. Other materials absorb light through other processes, such as changes in the vibrations of molecules, or in their rotations, etc. These processes show similar transitions to atomic transitions, but these may have much wider wavelength acceptance.

(Another thing to consider: photons do not have a precisely defined wavelength due to Heisenberg uncertainty principle)

As you can see the spectrum at the top of the atmosphere is continuous, with some saw tooth excesses, but still continuous.

The absorption does create a saw tooth pattern, even so there is continuity.

To dispel doubts here is the sun spectrum showing continuity and absorption spectra

Solar spectrum with Fraunhofer lines as it appears visually.

The first figure has been done registering intensities at each wavelength.