Fire element = lightning damage + radiant damage + fire damage
Electromagnetism is weird in D&D.
• The electromagnetic radiation is "radiant".
• The "electro" is "lightning".
• And the magnetic is "force".
Where the four alchemical elements correlate with states of matter, the alchemical element of "fire" correlates with plasma.
Medieval texts describe "the fire of the heavens", "the fire that burns fire", and even invisible "dark fire". These are surprisingly apt descriptions of plasma. The sun and stars are plasma, also lightning to some degree. The socalled dark fire corresponds to the invisible plasmosphere around the planet of Earth, between the earth and moon.
Plasma is when protons lose their electrons, thus becoming ions with electrical charge, but existing in a somewhat stable state. The behavior of plasma responds mainly to any electrical and magnetic fields.
Thus in D&D, plasma has qualities that relate "lightning" as well as the sun, even tho sunlight itself is photons relating to "radiant".
For this reason, I equate the D&D element of "fire" with several damage types:
Fire element =
• fire damage (heat burns, hot substance includes thermal radiation of photons)
• lightning damage (electricity and ionic plasma)
• radiant damage (photons, laser)
Note nuclear radiation includes photons and electrons and ions, thus is both lightning and radiant.