I disagree with the point stating he couldn't use light as a weapon. The spirit bomb gathered energy from all sources including the sun. Solar flare is also a use of the literal light. Not to mention huge damage buffs from kaio ken and super saiyan power increases. :)
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See, don't equivocate "Light" with other forms of light as we know it. The Guardians energy known as "Light" has a tangible and physical aspect to it. It is apparent it isn't just photons and electromagnetic waves. A solar flare is superheated material and radiation. Since people don't get sick by hanging with guardians (as far as we know) then it isn't radiation either. So the "Light" from Guardians is different.
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Thats a good point and I agree to a degree. The light of the guardians is manifest in elemental energy though. Goku's capacity to facilitate multiple energy sources for one attack would indicate, in my supposition, a similar if not the same use of energy. I think its somewhat a matter of semantics.
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Could be. We only have our best guesses to work with. The struggle is they both use unique forms of energy to accomplish their goal, Light vs Ki, and we have no direct comparison.