It looks like a coincidence and it is not. Energy and wavelength are tied by E = hc/λ, and hc — Planck's constant times the speed of light — happens to come out as 1239.84 eV·nm. Both of those constants have been exact by definition since the SI redefinition in 2019, so 1240 is not a measurement that might improve. It is a rounding, and it is good to one part in 1,300.
That is why a 620 nm red photon is 2 eV, why silicon with its 1.1 eV band gap stops responding past about 1,100 nm, and why anyone working in optics can convert between the two in their head.
It is vacuum wavelength only. Put the same photon in glass and its wavelength shortens by the refractive index — about a third in a typical crown glass — while its energy does not change at all. Feed the glass wavelength into 1240/λ and you get an energy the photon never had. Frequency is the safe quantity to carry across a boundary, because frequency is what stays fixed.
And it is per photon, not per beam. A 1 mW green laser and a 1 mW radio transmitter deliver the same power; the laser does it with about 3 × 1015 photons a second and the transmitter with roughly 1025. Nothing about the total tells you what a single photon can do when it arrives, and for anything to do with breaking bonds it is the single photon that matters.
The spectrum is one continuous thing. Nothing changes at 700 nm except that your eye stops responding, and the lines drawn on every textbook chart are a mixture of human physiology, engineering practice and historical accident.
Visible is defined by the observer, not the light — the usual 380–700 nm is a convention, and real eyes keep a little sensitivity past both ends. X-ray and gamma actually overlap, and the distinction between them is not wavelength at all: gamma rays are the ones emitted by an atomic nucleus, X-rays the ones from electrons. A 100 keV photon is an X-ray or a gamma ray depending entirely on where it came from, which is why this tool labels that region by origin as well as by number.
The one boundary that is physical is ionisation, and even that is a threshold rather than a line — it depends on what is being ionised.
This page has changed in 1 archived release, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.