Any creature can attempt to disbelieve an illusion. In order to do this, the creature must have sensed some hint that the subject may be an illusion (such as a fire not crackling, or an ogre not reacting to being struck by a sword). Disbelieving takes 1 round and causes the DM to make a private saving throw for the creature (or character) against spells, which does benefit from any WIS-based mental save modifier. If the save passes and the subject was indeed an illusion, then it is realized to be such by the saver.

If a creature disbelieves something, they do NOT get a save against it if it turns out to be real! For example, a trap would automatically strike, or a dragon's breath would automatically do full damage. Also, attacks from the subject (if it is actually real) automatically hit.

Illusions do not cast light (or darkness) in and of themselves. The creator of the illusion must have the illusion depict the effect of light. They can't use an illusion of light to actually illuminate a room and reveal what's in the room.

Imagine an illusionist is in a completely dark hallway and they create an illusion of a flame. If the illusionist doesn't know what's in that hallway, the best they can do is invent an illusion of an illuminated hallway. If the illusionist passed that way before with a real light, the illusionist could manifest an illusion of an illuminated hallway that looks like the real hallway. But in no case can the illusion truly illuminate the hallway. The illusory hallway and its contents lit with illusory light will be visible, but the real hallway and its contents will not be visible.