Can an illusion block a monster's harmful gaze?

Question

I'm curious of how illusions would affect spells or abilities that require a target to be looking at something. A good example of this would be Medusa's Petrifying Gaze which has the following effect:

Petrifying Gaze:

When a creature that can see the medusa's eyes starts its turn within 30 feet of the medusa, the medusa can force it to make a DC 14 Constitution saving throw if the medusa isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. [...]

So if someone were to cast Minor Illusion to put a bag over Medusa's head, would other player/creatures who do not realize that the bag is an illusion be affected by the gaze if they were looking in the direction of it? Does the illusion block the effect?

Answer 1

Yes

An illusory wall is just as effective at blocking sight as a real wall.

However, once the sight blocking properties of the illusion are no longer valid for a particular creature then it is no longer useful for blocking sight. This depends entirely on the specific illusion - a Minor Illusion would be useless after physical interaction or a successful Intelligence (Investigation) check, a Mirage Arcana would require truesight.

As an aside, a bag over the medusa's head would be next to useless as the medusa is immediately physically interacting with it and therefore knows the bag is an illusion. She (it?) can move her head outside the bag on her next turn.