Illusions, lighting, and reflectance

[Alexander Mont]

1. Alice is standing in a sealed corridor with a light on the other end. Bob the Illusionist, standing at the other end of the corridor, casts a Minor Illusion (or Silent Image, or something) to create the image of a wall that covers the entire cross-section of the corridor. Does this mean that Alice is now standing in darkness because the wall blocks the light?

2. If not, then consider a more normal case where Alice is looking at Bob in a large room and Bob casts a minor illusion of a wall in front of him to give him cover. Presumably in this case the illusion WOULD block the light that's reflected from the light source off of Bob (if the illusion generated its own light but didn't also stop the light reflected off of Bob, then Alice would just see the wall superimposed on Bob (and the background), since she's seeing both the light from the illusion and the light reflected off of Bob. So does that mean that the illusion can "tell the difference" between light reflected off of Bob and light coming directly from the light source, so it will block one but not the other?

3. If so, consider scenario (1) again but this time, the whole corridor is lit by a light source that is far away, and whose light is reflected off, say, a disco ball at Bob's end. Thus every ray of light that gets to Alice's end of the corridor (without the illusion) was reflected off of some surface at Bob's end. Does this change the answer to question (1)?

4. Can you make an illusion of a mirror, and will it reflect light normally? So you could make an illusionary mirror to peer around corners?

5. If illusionary objects don't reflect light normally, then the only way illusions could work at all is if they *gave off* light that matched whatever a viewer would actually see when looking at it if it were real. Under that interpretation an illusionary mirror placed across a corner would still show what's around the corner.

6. Another interpretation that I've seen is that the illusionist decides what the mirror shows - i.e. the illusionist is creating the image. This would imply effectively that (even in a normal case) the illusionist is figuring out what the illusion should look like given the ambient lighting and creating an illusion that shows that. (In other words, the illusionist has repurposed part of his or her brain as a GPU.) Note that this would mean that the illusion would still need to change as ambient light changed (and the illusionist would have to be initially aware of the positions of any light sources, especially if the object the illusion was being made of exhibited high levels of specular reflection).

7. If you know that an illusion is not real (e.g. because the illusionist who made it told you, or because you saw something pass through it) but you didn't actually spend the action to make the check to examine it, is it still faint/translucent for you? (What if someone who claims to be the illusionist tells you it's an illusion, and it actually is an illusion, but the person who told you isn't the illusionist? If this counts as knowing, it would be easy to get around illusions by having your friend "claim to be the illusionist" and "tell you it's an illusion" every time you see something that might be an illusion.)

 

[Terry Herc]

I feel that a minor illusion wouldn't replicate light effects, and that would be what distinguishes it from a major illusion. If this sort of situation came up in my game, I would read the spell language carefully and adjudicate based on that.

 

[Harzel]

Even though the spell description says "You create a sound or an image of an object", which makes it sound like you are creating a physical effect, I think it is much more reasonable to treat Minor Illusion and other illusion spells as mental effects, not physical ones. In support of this, consider

8. Alice and Bob are exploring a dungeon. Alice, in the lead, turns a corner and enters a room that Bob cannot yet see into. She finds an illusory object, interacts with it, and discovers it is an illusion. Bob now enters the room and sees the illusion. If we treat the illusion as a physical effect (as the OP does), then light reflecting from objects behind the illusory object is both passing through the illusion (for Alice) and not passing through the illusion (for Bob). OTOH, as a mental effect, the situation is a lot more plausible. The light in the room is bouncing around just as if there was no illusion. Alice's and Bob's perceptions of the illusion have nothing to do with the light that is incident on their eyes; rather the illusion magic is messing with their perception of the scene. Alice is, since her interaction with the object, able to partially resist the effect, while for Bob it is, for the moment, in full force.

Being a mental effect, an illusion need not conform to the laws of physics for illumination, sound propagation, or anything else, not even those of your fictional world. However, the greater the disparity between the situation presented by the illusion and what a character would normally expect, the more chance that I will grant bonuses to the INT(Investigation) check, or perhaps not even require the check.

 

[Ovinomancer]

1. Alice is standing in a sealed corridor with a light on the other end. Bob the Illusionist, standing at the other end of the corridor, casts a Minor Illusion (or Silent Image, or something) to create the image of a wall that covers the entire cross-section of the corridor. Does this mean that Alice is now standing in darkness because the wall blocks the light?

No. The illusion cannot create or block light or sound. It is a superimposed image only.

2. If not, then consider a more normal case where Alice is looking at Bob in a large room and Bob casts a minor illusion of a wall in front of him to give him cover. Presumably in this case the illusion WOULD block the light that's reflected from the light source off of Bob (if the illusion generated its own light but didn't also stop the light reflected off of Bob, then Alice would just see the wall superimposed on Bob (and the background), since she's seeing both the light from the illusion and the light reflected off of Bob. So does that mean that the illusion can "tell the difference" between light reflected off of Bob and light coming directly from the light source, so it will block one but not the other?

No. See above.

3. If so, consider scenario (1) again but this time, the whole corridor is lit by a light source that is far away, and whose light is reflected off, say, a disco ball at Bob's end. Thus every ray of light that gets to Alice's end of the corridor (without the illusion) was reflected off of some surface at Bob's end. Does this change the answer to question (1)?

No.

4. Can you make an illusion of a mirror, and will it reflect light normally? So you could make an illusionary mirror to peer around corners?

Illusions show what their caster can describe and only 8n thier area of effect. I would allow An illusion to reflect accurately within it's area of effect.

5. If illusionary objects don't reflect light normally, then the only way illusions could work at all is if they *gave off* light that matched whatever a viewer would actually see when looking at it if it were real. Under that interpretation an illusionary mirror placed across a corner would still show what's around the corner.

Illusions are magic, not physics.

6. Another interpretation that I've seen is that the illusionist decides what the mirror shows - i.e. the illusionist is creating the image. This would imply effectively that (even in a normal case) the illusionist is figuring out what the illusion should look like given the ambient lighting and creating an illusion that shows that. (In other words, the illusionist has repurposed part of his or her brain as a GPU.) Note that this would mean that the illusion would still need to change as ambient light changed (and the illusionist would have to be initially aware of the positions of any light sources, especially if the object the illusion was being made of exhibited high levels of specular reflection).

The illusion is still magic, it does a lot of this work for the caster.

7. If you know that an illusion is not real (e.g. because the illusionist who made it told you, or because you saw something pass through it) but you didn't actually spend the action to make the check to examine it, is it still faint/translucent for you? (What if someone who claims to be the illusionist tells you it's an illusion, and it actually is an illusion, but the person who told you isn't the illusionist? If this counts as knowing, it would be easy to get around illusions by having your friend "claim to be the illusionist" and "tell you it's an illusion" every time you see something that might be an illusion.)

No, you must make the INT check. Perhaps you could apply advantage to tge check if the caster tells you it's an illusion.

FWIW, I hate fading illusions and houserule that out. Illusions are visible regardless of the check. The check does allow you to determine the scope and extent if the illusion, but not see through it.

 

[jgsugden]

It would take 10,000,000 words to create thorough illusion rules. Not. Gonna. Happen.

Step 1: Ask your DM. They'll make a ruling.
Step 2: Ask about inconsistency with a prior ruling. Accept the response that magic is mysterious.

 

[Henry]

I would agree with [MENTION=16814]Ovinomancer[/MENTION] except for #1: an illusion would HAVE to block light in order for it to work at all. Not necessarily reflect it or create it, but block it absolutely, otherwise EVERY illusion would "appear faint" as the description for having made a successful Investigation check against the effect.

However, that corridor Alice is standing in would have to be no bigger than 5 ft x 5ft or light would go past the 5 ft cube.

In the case of mirrors and the like, it would have to have a reflection based on what the caster made the reflection look like, meaning it may have a "reflection", but it would not be dynamic.

 

[Ovinomancer]

I would agree with [MENTION=16814]Ovinomancer[/MENTION] except for #1: an illusion would HAVE to block light in order for it to work at all. Not necessarily reflect it or create it, but block it absolutely, otherwise EVERY illusion would "appear faint" as the description for having made a successful Investigation check against the effect.

However, that corridor Alice is standing in would have to be no bigger than 5 ft x 5ft or light would go past the 5 ft cube.

In the case of mirrors and the like, it would have to have a reflection based on what the caster made the reflection look like, meaning it may have a "reflection", but it would not be dynamic.

Magic. Illusions always add to a scene, they never subtract (unless specific, as with invisibility). This doesn't mean they can't conceal things, just that they can't remove things. Illusions cannot remove existing light. If the crafted illusion attempts to it's flawed and more easily detectable. I give the benefit of the doubt that illusions are created in ways that minimize such discrepancies, such as a wall covering a light source will itself have an apparent source for the light.

 

[Laurefindel]

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.

 

[Swarmkeeper]

...I give the benefit of the doubt that illusions are created in ways that minimize such discrepancies, such as a wall covering a light source will itself have an apparent source for the light.

Yes to this. Solves all the white room musings about illusions blocking light, etc. The light can appear to come from the illusion for those that are fooled, but it has its own real source.

 

[FrogReaver]

I think if you have an illusion and it doesn’t behave nearly identical in all ways to the regular object then it’s not really a magical illusion. I think There can be no consistent scientific Phenomenon that can be observed or not observed to indicate if something is an illusion. However each illusion has small flaws that when carefully observed can indicate its an illusion. If those flaws were consistent then all a character would need to do is look for such phenomenon and it would indicate you are viewing an illusion. That clearly isn’t the case.

So illusory mirrors reflect. Illusory walls block light. Etc. once you know it’s an illusion the magic is broken and the light passes through and the reflections stop etc.