According to RAW what happens to an illusion when it is identified?

Question

A player recently insisted that if a character identifies that something is an illusion, then that illusion becomes translucent to the character.

The example was given that the wizard could make an illusion to hide the party, but because they know it's an illusion they can all see through it, but enemies the other side can not.

This runs differently to how I have always run illusions, yes your character knows it is an illusion but it is still solid and still can't be seen through,

I have tried reading through the rule book to find the RAW approach but can't find it, is there one?

Answer 1

The source of the illusion will tell you if its appearance changes when detected.

The player is probably recalling one of these lines from minor illusion, silent image, major image, or project image:

If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.

If a creature discerns the illusion for what it is, the creature can see through the image.

If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.

If a creature discerns the illusion for what it is, the creature can see through the image, and any noise it makes sounds hollow to the creature.

These spells create illusions and then describe what a creature sees if they successfully discern the illusion, but not all features that create illusions do this. For example, the illusion created by the spell mirage arcane can be seen through with truesight, but the spell description says nothing about what a creature sees if they know about the illusion but do not have truesight; so we conclude that knowing a mirage arcane is an illusion does not change its appearance for you.