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https://en.m.wikipedia.org/wiki/Rydberg_formulaAs stressed by Niels Bohr,[2] expressing results in terms of wavenumber, not wavelength, was the key to Rydberg's discovery. The fundamental role of wavenumbers was also emphasized by the Rydberg-Ritz combination principle of 1908. The fundamental reason for this lies in quantum mechanics. Light's wavenumber is proportional to frequency 1/L=f/c, and therefore also proportional to light's quantum energy E. Thus, 1/L=E/hc (in this formula the h represents Planck's constant).

So there's no voltage where the light suddenly "turns on".

Oh yes there is!

Above that value the intensity of emission increases with current, usually limited by heating which reduces efficiency, but the spectrum is relatively fixed.

Returning to the original question, EMR is not necessarily quantised. Maxwell's propagation laws apply at any and all frequencies, so you can in principle generate radiation at any photon energy you want.

Since the lowest visible energy is about 1.2 eV...

Afaik, Maxwell's model for electromagnetic radiation has no concept of photon.

Where does the photon energy come from

Thermal energy can make a contribution to promoting an electron into an excited state. (which is also why the V/I curve is temperature dependent.)

Quote from: hamdani yusuf on 29/05/2023 15:12:23Afaik, Maxwell's model for electromagnetic radiation has no concept of photon.But ours does. And Maxwell doesn't have a model - he derived a selfpropagating wave from the mathematics of known experimental electromagnetic phenomena.

one authoritative source calculates 5 eV

In his model, light is propagating electromagnetic wave in a medium.

conservation laws suggest

Returning to the original question, EMR is not necessarily quantised. Maxwell's propagation laws apply at any and all frequencies, so you can in principle generate radiation at any photon energy you want. What Planck said was IF you have an ideal particle rattling about in a perfectly elastic box, it can only have discrete energy levels, and you can use this model to predict the UV spectrum etc. If the box does not have defined dimensions then the number of permissible states tends to infinity, hence the black body continuum with a continuous distribution of energy versus wavelength as predicted by the rigid box model for any particular wavelength.

https://en.wikipedia.org/wiki/History_of_quantum_mechanics#Founding_experimentsFounding experimentsThomas Young's double-slit experiment demonstrating the wave nature of light. (c. 1801)Henri Becquerel discovers radioactivity. (1896)J. J. Thomson's cathode ray tube experiments (discovers the electron and its negative charge). (1897)The study of black-body radiation between 1850 and 1900, which could not be explained without quantum concepts.The photoelectric effect: Einstein explained this in 1905 (and later received a Nobel prize for it) using the concept of photons, particles of light with quantized energy.Robert Millikan's oil-drop experiment, which showed that electric charge occurs as quanta (whole units). (1909)Ernest Rutherford's gold foil experiment disproved the plum pudding model of the atom which suggested that the mass and positive charge of the atom are almost uniformly distributed. This led to the planetary model of the atom (1911).James Franck and Gustav Hertz's electron collision experiment shows that energy absorption by mercury atoms is quantized. (1914)Otto Stern and Walther Gerlach conduct the Stern?Gerlach experiment, which demonstrates the quantized nature of particle spin. (1920)Arthur Compton with Compton scattering experiment (1923)Clinton Davisson and Lester Germer demonstrate the wave nature of the electron[27] in the electron diffraction experiment. (1927)Carl David Anderson with the discovery positron (1932), validated Paul Dirac's theoretical prediction of this particle (1928)Lamb?Retherford experiment discovered Lamb shift (1947), which led to the development of quantum electrodynamics.Clyde L. Cowan and Frederick Reines confirm the existence of the neutrino in the neutrino experiment. (1955)Clauss J?nsson's double-slit experiment with electrons. (1961)The quantum Hall effect, discovered in 1980 by Klaus von Klitzing. The quantized version of the Hall effect has allowed for the definition of a new practical standard for electrical resistance and for an extremely precise independent determination of the fine-structure constant.The experimental verification of quantum entanglement by John Clauser and Stuart Freedman. (1972)The Mach?Zehnder interferometer experiment conducted by Paul Kwiat, Harold Wienfurter, Thomas Herzog, Anton Zeilinger, and Mark Kasevich, providing experimental verification of the Elitzur?Vaidman bomb tester, proving interaction-free measurement is possible. (1994)