D&D 5E Minor Illusion question

[Lyxen]

This is only an issue if your world features real world physics and non-smart magic (ie, magic that operates within physics like technology rather than creating effects based on intention).

Good answer. Magic is magical by definition, and light in D&D certainly does not behave like light in the real world (otherwise you would, for example, be blind while being invisible, assuming that invisibility could even be explained in terms of real world science, which I don't think is possible).

Minor illusion cannot work at all according to real world physics anyway if you think about it properly (you never see an object, you only see the light reflected off it, so if you don't create an object but just an illusion, you are probably only creating light, but then why wouldn't you be able to create a light source, as a first question).

 

[Dausuul]

Consider the case of someone creating an illusion of a wall just in front of themselves to hide their presence. If visual illusions can't interact with non-illusory light, this does nothing, as light will pass through the illusion, be reflected by the caster, pass through the illusion again, and finally enter the eyes of anyone in the area. We can reason by extrapolation that this would make nearly all visual illusions ineffective. Therefore, visual illusions must be able to interact with light, if we want the spells to do anything at all.

Except that a person who makes their Investigation check sees the illusion as a transparent outline. So light must be able to pass through it. I see two possible explanations:

• The illusion works on the mind of the observer. Nothing at all is happening to the physical photons. They hit the eye normally, but the information gets lost somewhere in the visual cortex.
• There is a quantum interference kind of thing going on, where photons simultaneously pass through the illusion and reflect off it. The interference is resolved when a believer or nonbeliever steps into the photon's path.

While the second explanation appeals to my sense of geeky aesthetics, the first is a lot easier to reason about.

(Bonus question: If an invisible person is standing in between you and the illusion, such that a straight line can be drawn through your eyeballs, their eyeballs, and the illusion... who sees what?)

 

[Fenris-77]

Magic is magical is about all you need here. Let's not overthink our elf games.

 

[Sword of Spirit]

Except that a person who makes their Investigation check sees the illusion as a transparent outline. So light must be able to pass through it. I see two possible explanations:

• The illusion works on the mind of the observer. Nothing at all is happening to the physical photons. They hit the eye normally, but the information gets lost somewhere in the visual cortex.
• There is a quantum interference kind of thing going on, where photons simultaneously pass through the illusion and reflect off it. The interference is resolved when a believer or nonbeliever steps into the photon's path.

While the second explanation appeals to my sense of geeky aesthetics, the first is a lot easier to reason about.

(Bonus question: If an invisible person is standing in between you and the illusion, such that a straight line can be drawn through your eyeballs, their eyeballs, and the illusion... who sees what?)

This is both a fun thought experiment and a perfect example of why I insist in my campaign that D&D doesn't follow real world physics (or biology, or chemistry). No atoms, no photons, no cells. Microscopes don't exist, but if they did and you looked at something at what should be a cellular level you'd either see a field of color that looks like what you see on the macro scale, or perhaps swirling elemental energies.

And see the original Spelljammer boxed set for the explanation of how space and gravity work.

 

[Dausuul]

Magic is magical is about all you need here. Let's not overthink our elf games.

It's all very well to say that, but it doesn't actually answer the question of whether the illusion can hide torchlight (and, if not, what does happen).

I find that applying real-world physics is very helpful in coming up with a consistent model of what's going on, so I don't tie myself in knots with off-the-cuff rulings that I can't keep straight from one session to the next.

 

[Lyxen]

It's all very well to say that, but it doesn't actually answer the question of whether the illusion can hide torchlight (and, if not, what does happen).

It's your game, so you decide for your game (rulings over rules). As for me, I would rule that since you created the illusion of an object that people believe in, it would be really silly if light could shine through it, so yes, it would block light. The fact that you can see through it if you determine that it's an illusion still does not mean that it does not block light, in D&D vision and light are not linked like they are in the real world, again because of magic.

I find that applying real-world physics is very helpful in coming up with a consistent model of what's going on, so I don't tie myself in knots with off-the-cuff rulings that I can't keep straight from one session to the next.

As demonstrated, real world physics can only take you so far in a world full of magic which breaks the most fundamental of physics principle. Gravity does not behave as real world gravity (spelljammer gravity is official), people do not breath N2/O2 melange but the stuff from the elemental plane of air, and fireball are not combustion since they don't need air and produce no residue so they are probably made from stuff from the plane of fire. In such an environment, real world physics, especially complex and very misunderstood theories like quantum physics have very little value.

 

[Fenris-77]

It's all very well to say that, but it doesn't actually answer the question of whether the illusion can hide torchlight (and, if not, what does happen).

I find that applying real-world physics is very helpful in coming up with a consistent model of what's going on, so I don't tie myself in knots with off-the-cuff rulings that I can't keep straight from one session to the next.

I personally couldn't care less about real world physics when it comes to things like illusions. Everyone has different tolerances there, of course. For me though, as soon as we start talking about does the illusion mask torchlight we've probably jumped the shark. It's not much of an illusion if we can nitpick the physics to death, and I have absolutely zero interest in doing that in my games. YMMV. So, anyway, that's me and my game.

 

[Dausuul]

For me though, as soon as we start talking about does the illusion mask torchlight we've probably jumped the shark.

That's the difference between a novel and an RPG: These questions come up in RPGs and can't be waved off as shark-jumping. Because one of the PCs just cast minor illusion on a torch, and now the DM needs to say what happens. And it is generally preferable for the DM's answers to be consistent so the players don't find their abilities changing randomly from one session to the next.

That's where I find real-world physics helpful, not because the D&D world necessarily operates that way, but because it helps me frame the question in my head. Thinking about the movement of actual photons gives me a way to think through the consequences of any given interpretation. For example, if I rule that the light is visible even though the torch is not, that raises the question of whether illusions cast shadows, and if so, at what point "object casting a shadow in torchlight" becomes "barrier that the torchlight can go through."

(The quantum stuff was just an aside. I was in the middle of explaining why "it's all in your head" is the only consistent way to run it, and that's when it occurred to me that you could do a weird dual-realities thing instead where the illusion is both there and not-there. As I said, the "it's all in your head" explanation is much easier to reason about. But, again, I would never have thought of "there/not there"--which actually feels very mysterious and magical, and might be worth exploring--without my sketchy layman's knowledge of quantum physics.)

 

[Fenris-77]

Of course they can be waved off as shark jumping. I just did it. You might not want to, and that's cool, but I have no interest in parsing the deep sweaty depths of how illusions work, so I mostly just adjudicate them as advertised.

 

[Sword of Spirit]

It's all very well to say that, but it doesn't actually answer the question of whether the illusion can hide torchlight (and, if not, what does happen).

I find that applying real-world physics is very helpful in coming up with a consistent model of what's going on, so I don't tie myself in knots with off-the-cuff rulings that I can't keep straight from one session to the next.

I‘d go with saying an illusion of an object hiding a light source also blocks the light, just as it would block vision of an object. The light being shed is part of the light source in this case. That seems like the most intuitive way you would expect the magic to work. A creature who can see through the illusion sees that light also. I guess that means it has a partially mental component.

I do like how 3e had various subtypes of illusion, such as figments, glamours, phantasms, and shadows that all worked differently ways. In 3e though, having a mental aspect meant some creature weren’t affected by it the same, where 5e only applies that to spells that are fully in the mind of the viewers. Your dual-reality idea is a better way of dealing with it, but I’d simple treat the different realities as the physical reality verses the perceived reality. Rather than it being all in your head, the perceives reality is objectively presented to the world, and even mindless perceivers experience it. Those who see past it get to see both realities simultaneously.

The main determinant of how the perceived reality works could be to make the spell accomplish its goals with minimal negative side effects or extra benefits. A torch behind a box shedding light doesn‘t accomplish the goal of the spell. A creature not having a shadow also doesn‘t accomplish the goal. Invisibility that blinds you and maintains your shadow definitely doesn‘t accomplish its goal. At the same time, an illusion that walls off a window, blocking the sunlight coming in and placing the whole room in darkness is granting a potential benefit not consistent with the spell’s function. I’d say that the perceived reality would still block any direct beams of light, but it would not affect the overall ambient level of light in the room. So the perceived room would be ambiently lit but experimentation would only reveal the source of the light if you passed the Investigation check (and thus saw through the illusion and could see both realities). Otherwise the light would appear to emanate from the air itself in a way that minimizes suspicion of the actual window location. Illumination would therefore be more of a “perceived” rather than physical thing in D&D.