Why light spells are the most efficient and least complex form of magic.
Before I begin, a simple explanation must be given.
Mana is a form of Stored Chemical energy produced in the bone marrow of the body along with blood cells, Chemical energy can easily be converted to Electromagnetic energy by a process called electron shifting, chemical energy is actually a form of potential electromagnetic energy.
For the reason why Light magic is the easiest form of magic to use, Light is Electromagnetism, so basically casting a light spell is just converting Energy between the Chemical(potential electromagnetic energy) to actual Electromagnetic energy, which is a very easy conversion.
And thus in the System made by AHIH, Mana→Photon, spell has the base complexity with an L of 1, L meaning complexity of the spell.
Complexity is defined by how many physical laws the spell obeys, the more laws it obeys the less the complexity to a base of 1.
Light magic is also highly dangerous to humans, due to lack of complexity one could quite easily create Gamma Radiation, since gamma waves are just high energy Electromagnetic waves.
But it becomes ineffective and even strengthening against creatures that like the Ahih race or Dragons, an Ahih can absorb any form of energy and convert it into usable one, Dragons have ancestors that evolved before the ozone layer of this world developed thus they have resistance to radiation.
And due to radiation light magic researchers always seem to die fast after beginning experimentation to it, due to this light magic got the stigma that it is divine and that only the blessed few could use it without retaliation from the divines, of course, this is just philosophy, only Lehatot has Authority over the Light domain, or rather electromagnetism and the reason is obvious.
Currently almost no mages opt into the light domain, and almost no school teaches it.