Illusions, lighting, and reflectance

[Keravath]

As Ovinomancer said, illusions cannot create light, block light, or reflect light. Otherwise, it reacts exactly like a normal object would, as long as it is confined within the area of effect of the spell.

I know it doesn't make sense from a physics point of view, but once you accept this as true, the rest is easy(er) to deduce.

So Alice would not be in darkness behind her illusory. If she were the bearer of the torch, the illusion would not prevent the corridor from being illuminated (although Alice and the source of light itself would be hidden behind the illusory wall).

Since, an illusory object reacts as a real object, a disco ball effect would appear on the illusory object as if it were real, but the illusory object itself would not reflect dots of light on other items outside its area of effect.

There are times where an illusion can appear to break the three rules above (can't produce, block or reflect light), but the key word here is "appear". Illusion are about appearance. For example, an illusory item could cast a shadow (even though shadows are by definition light being blocked) because a real object would; but the shadow would need to be confined to the area of effect. Likewise, an illusory mirror would reflect the image of those who look into it, because that's what a mirror does, but it could not be used to reflect the light of the sun inside a cave for example, or send a signal to a far-away place.

TL;DR: don't involve physics or optics. Everything about the illusion looks real within its AoE, but you can't cheat the spell to create a source of light when there is none, make darkness where there is none, or reflect light to a point beyond its AoE.

I think illusions have to come down to accepting that they are magic and result in unphysical consequences. The rules specifically say that the illusion doesn't create light.

However, if an illusion doesn't at least APPEAR to block and reflect light then it won't look like a real object.

If you cast an illusion of a chair and it doesn't include an appropriate shadow then it really WON'T look like a chair .. it will look like some weird image that would not be able to convince anyone that it was a chair. If you cast an illusion of a wall or a tree and the lighting pattern on the object is static as light sources move or chance then again it will be extremely obvious that it isn't real. Obvious enough that an action to inspect it would not be required ... it would be immediately dismissed as not real.

What about a chair sitting in the sun near sunset? It will be obvious what it is if the shadow it casts abruptly cuts off after 5'.

"Physical interaction with the image reveals it to be an illusion, because things can pass through it."

In reality if you observed the photons of light passing through the object and reflecting off the surfaces behind the illusionary object (which would ALWAYS happen) ... then by RAW the image would be revealed to be an illusion since LIGHT itself is a physical interaction with the image which you can observe. However, this interpretation would make illusions utterly and completely useless and likely not intended.

So .. it comes back to magic. Either an illusion is created in the mind of the creatures affected which results in them seeing something and their mind adds all the details and interactions that make the object seem more or less real OR the illusion creates a physical manifestation that also has some properties like interacting with light at a minimal level to make the illusion seem real. Without some aspect of realistic environmental interaction with light ... (or the equivalent in terms of magically replicated apparent interactions) ... illusion magic just doesn't work.

 

[Laurefindel]

I think illusions have to come down to accepting that they are magic and result in unphysical consequences. The rules specifically say that the illusion doesn't create light.

Agreed

However, if an illusion doesn't at least APPEAR to block and reflect light then it won't look like a real object.

Agreed again

If you cast an illusion of a chair and it doesn't include an appropriate shadow then it really WON'T look like a chair .. it will look like some weird image that would not be able to convince anyone that it was a chair. If you cast an illusion of a wall or a tree and the lighting pattern on the object is static as light sources move or chance then again it will be extremely obvious that it isn't real. Obvious enough that an action to inspect it would not be required ... it would be immediately dismissed as not real.

Agreed. However, I doubt that in a room full of furniture, a chair that doesn't cast a shadow would be that obvious at first sight. After a successful investigation check, yes it would, but that what the investigation check is for.

But assuming a minor image cantrip, there is enough space in the AoE to include a shadow. Except it will be the appearance of a shadow, like the ground will be of a darker shade there, but the amount of light would be unchanged.

We see it all the time in trompe-l'œil paintings, or the street art that is designed to create an optical illusion.

Could a character find something weird about the chair and its shadow by observing how light react with it? Perhaps, enough to warrant a INT save, but it won't be noticeable by casually walking by it.

What about a chair sitting in the sun near sunset? It will be obvious what it is if the shadow it casts abruptly cuts off after 5'.

If it was the only object in a white room, yes it would be obvious. However, this is rarely the case, and I doubt anyone would casually notice. If they start looking for telltale signs of illusions, they'll probably have a lower DC on their investigation check. And if the caster wants to be safe, use a silent image spell instead. The 15' cube ought to be enough for most situations. Minor image is, after all, just a cantrip. It's ok if that spell isn't 100% foolproof.

"Physical interaction with the image reveals it to be an illusion, because things can pass through it."

In reality if you observed the photons of light passing through the object and reflecting off the surfaces behind the illusionary object (which would ALWAYS happen) ... then by RAW the image would be revealed to be an illusion since LIGHT itself is a physical interaction with the image which you can observe. However, this interpretation would make illusions utterly and completely useless and likely not intended.

Oh I know, that's why I said it's better to leave real-life physics out of magical effects. You have acknowledged that as the first sentence of your post.

The same that goes for shadows is also true for reflections. The illusory item appears to be lit just like a real item would, with lighter highlights, just like in a trompe-l'œil painting or optical illusion. An illusionary mirror would show your reflection if you looked into it, but it wouldn't bounce the light of your torch on the ceiling, unless the ceiling is within the area of effect. Again, the minor illusion spell has its limitations, its a cantrip. A 15' cube silently image has a better chance of being a better illusion. I'm ok with that.

So .. it comes back to magic. Either an illusion is created in the mind of the creatures affected which results in them seeing something and their mind adds all the details and interactions that make the object seem more or less real OR the illusion creates a physical manifestation that also has some properties like interacting with light at a minimal level to make the illusion seem real. Without some aspect of realistic environmental interaction with light ... (or the equivalent in terms of magically replicated apparent interactions) ... illusion magic just doesn't work.

It doesn't have to be a physical manifestation. It just has to be a magically manifestation. Illusions are intangible, therefore cannot reflect light (and thus would be invisible) with our conception of physics. If they could reflect or block light, they would necessarily need to be tangible. (perhaps bringing its own set of telltale signs?) I wouldn't have a big issue with that if it weren't for the fact that once you succeed on you INT save, the physical qualities of the illusory item changes (if only in opacity) for you but not fo those who didn't succeed. It's that quantum state that troubles me. Illusions, as described in D&D, cannot be reconciled with physics, they are going to break its laws in one way or another. Hence the incistance on the magical element of magic spells.

 

[Ovinomancer]

I think illusions have to come down to accepting that they are magic and result in unphysical consequences. The rules specifically say that the illusion doesn't create light.

However, if an illusion doesn't at least APPEAR to block and reflect light then it won't look like a real object.

If you cast an illusion of a chair and it doesn't include an appropriate shadow then it really WON'T look like a chair .. it will look like some weird image that would not be able to convince anyone that it was a chair. If you cast an illusion of a wall or a tree and the lighting pattern on the object is static as light sources move or chance then again it will be extremely obvious that it isn't real. Obvious enough that an action to inspect it would not be required ... it would be immediately dismissed as not real.

What about a chair sitting in the sun near sunset? It will be obvious what it is if the shadow it casts abruptly cuts off after 5'.

"Physical interaction with the image reveals it to be an illusion, because things can pass through it."

In reality if you observed the photons of light passing through the object and reflecting off the surfaces behind the illusionary object (which would ALWAYS happen) ... then by RAW the image would be revealed to be an illusion since LIGHT itself is a physical interaction with the image which you can observe. However, this interpretation would make illusions utterly and completely useless and likely not intended.

So .. it comes back to magic. Either an illusion is created in the mind of the creatures affected which results in them seeing something and their mind adds all the details and interactions that make the object seem more or less real OR the illusion creates a physical manifestation that also has some properties like interacting with light at a minimal level to make the illusion seem real. Without some aspect of realistic environmental interaction with light ... (or the equivalent in terms of magically replicated apparent interactions) ... illusion magic just doesn't work.

I don't see how the former is indicated at all but the rules, but the latter, I believe, is clearly indicated -- within the AOE. I will rule that illusions will cast shadows within their AOE and will appear to be appropriately illuminated by their environments. They won't add elements (or lose them), but to rule other than this makes illusions useless. This, however, doesn't require any interaction with physics or light in a manner defines by physics, it's just the magic of the illusion.

Full disclosure: I'm an electrical engineer. I make my living on light. I'm much happier explaining illusions as magical non-physics rather than playacting that my imaginary world follows the rules of physics. Doing that for illusions opens doors for questioning why light spells create a fixed radius of bright light which has a hard falloff to dim light which has a hard falloff to complete darkness. D&D is NOT the place for physics.

 

[niklinna]

My quantum take on illusions

There are two basic readings for illusions:

• Illusions primarily manipulate, and possibly create, light or other physical but specifically non-tactile effects such as sounds and scents (whereas phantasms strictly affect the target's mind).
• Illusions and phantasms primarily manipulate the senses and/or minds of creatures.

Both views are problematic, in the same way that the wave/particle duality of classical physics is problematic. And this thread is just talking about visual illusions.

Illusions have an area of effect for the effect, not for who is affected, whereas phantasms explicitly have particular targets who are affected. If I cast a Minor Illusion of a bright green 5' cube, everyone can see it who could see a 5' large object, and this "area of who is affected" is even bigger for higher-level and therefore larger illusions. This is quite a feat for something that affects the minds of individual creatures. And yet, nearly every illusion is described as becoming transparent/faint just for those who make their Investigation checks, which strongly implies that the effects are mental. It's self-contradictory and leads to illogical and inconsistent interpretations no matter how you try to reason things out.

Good thing we're talking quantum physi—er, magic—which is capable of defying all logic, hm?

So, for me the most parsimonious explanation is that a visual illusion of an object basically creates a localized field that bounces visible-frequency light off as the object being illused would. That means illusions can cast shadows and therefore block light. You might also rule that illusions block visible, but not infrared or ultraviolet light, so that you can feel the warmth from a fire behind an illusory wall, or get a sunburn while shading under an illusory tree, possibly including such frequencies by upping the spell level. (You could also extrapolate on this to consider how reptiles, birds, and insects, with visual abilities into the ultraviolet, are affected by illusions created by mammals, but that's up to you!)

Creating light means creating energy, however, and that's a different game. For the cantrip Minor Illusion, which is explicitly stated as not being able to create light, my thinking is that it's too weak to do more than bounce photons. By comparison, the Light cantrip can create such energy, but can't shape it in detail the way Minor Illusion can. Cantrips can do one or the other at their meagre power level. (Sound is in some ways less complex than light—at the very least it's two-dimensional rather than four—so I can see how Minor Illusion could create a sonic effect.)

The first level spell Silent Image is silent about whether it can create light, but does say "some other visible phenomenon", and light is a visible phenomenon, so I would allow illusion spells of first level or higher to create light, and would judge based on spell level just how much light they can generate. I would use light-generating spells of similar levels to adjudicate how that light behaves in terms of radius and falloff and whatnot (which, as others have noted, have their own physics-defying behavior).

Now, the rule about illusions becoming transparent/faint to those who make their Investigation checks, or such creatures being able to see through them (it's worded differently in different places), is where things really get quantum, as it were. This is a simple statement of fact in the rules, with no explanation given, so the simplest way to handle it is to say, okay, that's what happens, and not worry about an explanation. If an illusion becomes transparent or faint to somebody, it doesn't block light any more (or only partially), just for that somebody. Maybe by interacting* with the illusion-creating field, they create (or disrupt) some quantum or magical entanglement with their own substance that ruins the effect. Maybe there's some other reason. But the rules are explicit about what happens.

* And anybody who knows a little about Schrödinger's cat knows what a philosophical thicket even defining "interacting" can be. The rules do seem to be strongly implying that you need to touch the illusion, but that can only apply to visual illusions. How do you physically interact with a sound or a smell?

So yes, in my interpretation that means someone could be put in darkness by a large illusion blocking an entire corridor. This is roughly equivalent in power level to the Darkness spell, except this is normal darkness: it doesn't affect darkvision and is easily removed by creating your own light source or investigating the wall, whether by touch or by hearing clear sound coming from beyond it. So I don't have a problem with that.

To answer the specific questions then:

1. Yes, Alice is in darkness. Normal, nonmagical darkness. If she makes her Investigation check, she can see through it, and she is no longer in darkness, even though anybody else who hasn't made their check is.

2. The illusion blocks both light sources: The wall casts a shadow and blocks Bob from view (along with any light sources he may be holding). If Alice touches the wall or otherwise makes her Investigation check, she can see through the wall and, just for her, it doesn't cast (much of) a shadow.

3. No matter where the light comes from, or how many objects it is reflected from, if the illusion blocks it off from Alice, Alice is in darkness.

4. Yes, you can make an illusion of a mirror and it will reflect light normally, and yes you can use it to peer around corners. But anybody around that corner can also see the mirror and whatever it reflects (that is, you).

5. I've already addressed this above with the commentary on modifying vs. generating light. My answer remains as for #4.

6. This interpretation is perfectly valid for any illusion that the caster can change dynamically. You can create an illusion of something that looks like a mirror, and have it show whatever you want—with the proviso that it shows the exact same image to everybody, no matter their viewing angle. I don't think the illusionist is required to actively create a regular reflection, however, if a regular reflection is what they want.

7. Per the rules, for most illusion spells I have checked*, you have to make an Investigation check to determine an illusion is an illusion, and yet they are silent on foreknowledge, so I have to say this is up to the DM. By my interpretation above, some sort of interaction is necessary. If you like that interpretation, maybe you'll give anybody with foreknowledge a bonus or advantage on the Investigation check, but it still needs to be made for you to overcome the actual effects of the illusion.

* It's a bit silly that such text has been repeated in each illusion spell, leading to differences from spell to spell. A Minor Illusion, which is a cantrip, becomes faint, but for a Silent Image the creature can "see through" it.

Update: Under a strict interpretation of the above, illusions cannot do "cutaways"—pits or doorways/windows into illusory rooms—because the light would hit the material that's already there. A looser interpretation would allow for anisotropic two-dimensional illusions that mimic three-dimensional forms (to get a bit into the likely physics of things again). Somewhere in between, you could say that Investigation checks against such illusions are made with a bonus or at advantage, or you could require the ilusion spell to be cast at a higher level.