I was mostly referring to copycat effect of light spells (if illusion can create light), invisibility effects (if illusion can remove/bend/reflect light), and darkness shenanigans (if illusion can block/absorb light). I'm not saying you advocated any of those, but that's why I think one should be careful about allowing illusions to create, block or reflect light.
But again, as Ovinomancer said, illusions as described in D&D (and fantasy in general) are irreconcilable with real-world physics. Something's got to give. In the face of this paradox, we've got to accept that illusions won't logically work or be rationally explainable, and go with the same willing suspension of disbelief that we are capable of for many other aspects of the game.
In order to be visible in the real-world, an object as to a) emit in the visible spectrum, or b) reflect the ambient light toward the onlooker (assuming there is ambient light in the first place).
Illusions are magical constructs that can make something visible and look real enough to fool anyone that doesn't physically interact with it, but that doesn't really exist. So far in this tread, it has been theorized that...
- Illusions don't create, absorb or reflect light in any logical ways: it just make the illusory item visible and indiscernible from a real one, because magic.
- Illusions block light and reflect it back to it onlooker (the reason we see them) but don't interact with the physical world in any ways, because magic.
- Illusions don't exist as a phenomenon but as a figment of the imagination of the onlooker. The illusionist therefore doesn't mess with our senses as mush as it messes with our mind. And this figment is the same for everyone, because magic.
Personally, I don't think one explanation is more valid than any other - all require that we ignore the laws of physics as we know them. However, I find that the first one is the simplest to explain and to implement within the RaW parameters of the spell without too much surprises and dubious shenanigans. And yet it remains a very powerful tool in the hand of a clever illusionist.
I know I already posted this, but:
Solution:
Illusions (like thick, coloured smoke), are effectively insubstantial for beings/objects passing through them, and are composed of coloured gases that have been trapped within insubstantial planes of extremely refined (and rather detailed) magical charge. They still block light, and cast shadows, but can't be consider substantial (as they are basically coloured gas and magical fields of directing electric charge.