D&D 5E Reminders on Illusions

ECMO3

Hero
From reading, and taking literally, that an illusion cannot generate light.

Having the illusion cast a reflection means it's generating light. In particular, consider a situation where an observer can see the illusion but not the light source (maybe the light's around a corner, or the viewer is indoors but the illusion is outside "reflecting" the sun).
The problem with this is your eyes only see light.

When you look outside your window at the green leaves, what is actually happening is light from the sun is hitting the leaves. The light from the sun is "white" light and includes all visible colors as well as infrared and ultraviolet. All that light hits those leaves and the green portion is reflected into your eyes while the other colors are absorbed by the leaf. What you see is a reflection of the green portion of the sunlight.

When you look at yourself in a mirror the same thing is happening, light from a source (sunlight, lamp etc) is reflecting off of you. The colors that reflect off of you hit the mirror, the other colors are absorbed by your body. This light that is reflected off of you then reflects again off of you to the mirror, and back to the eyes of whoever is looking at the mirror. A mirror reflects all colors of visible light, so unlike your body or clothes or the leaves it does not absorb any of it and what you "see" is the exact same colors that hit the mirror to start with. Because it is a polished flat surface the reflection is clear and it maintains spacial coherency, unlike a leaf or something that is textured.

So bottom line, if the illusion does not create light and does not reflect light, then it can not be "seen" by your eyes.

Edit: This post assumes the leaves are green, I realize many of us are looking at red or brown leaves right now.
 

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gator001

Villager
Read the description of the spell again. It's not minor illusion that can't create sensory effects, it is the image created by minor illusion that can't.

Specifically is says: If you create an image of an object—such as a chair, muddy footprints, or a small chest—it must be no larger than a 5-foot cube. The image can't create sound, light, smell, or any other sensory effect.
How do you see an illusion?
 



Laurefindel

Legend
physics-bending magic is always hard to explain with rational physical explanations (or optical in this case).

The very nature of a (visual) illusion is to fool our sense of sight. Our sense of sight is made possible by light reaching our eyes, either by emanation or reflection (or refraction). A visual illusion can be seen, therefore it must be producing whatever it needs to produce in order to be seen and fulfill its role as an illusion. So in order to explain illusions realistically, they need to produce light. By the same logic, they can also absorb light, otherwise we'd be seeing through them.

But I believe that the spirit of the rule here is that a minor illusion cannot produce illumination, and that is simple enough to comprehend and implement. If you create an illusory (lit) candle in darkness, it won't produce illumination in a 5 foot-radius like a real lit candle would. As a matter of fact; it won't be seen by anyone that do not have darkvision. An illusory torch will not increase a dimly lit area to bright light either. Both will appear "lit" to an observer who can see them but the lack of illumination will be mind-boggling. Something will feel wrong, enough to warrant a Int save at the very least, if not an auto-detection as if interacted with.

The contrary is also true; realistically speaking, an illusion needs to block light in order to be opaque, but it cannot block "illumination". An illusory rock over a candle will obfuscate the candle, but it won't block the light produced by the flame that illuminates the area around it (well I guess a 5-foot illusion could cover the 5-foot illumination area of a candle). So you'd be stuck with a glowing rock; which can be weird enough to warrant an Int save or prompt some kind of interaction (which in turn will betray its illusory nature).
 
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jgsugden

Legend
A couple feedback items to people on this thread:

* Sage Advice comes from the people that approve the rules, and usually goes to clarifying what is intended. You can approach them as you like, but they are meant to convey the intent of the RAW.

* Real world physics are fine to consider until they contradict the rules. This is a fantasy game and involves fantasy elements. If the written rules for illusions create a situation that differs from the real world way that light and vision work, it is reality, not the rules of the game, that is wrong - because otherwise we'd be invalidating the entire rule set because none of the fantasy is reality.

* Some of the things I presented note an ambiguity, and the consensus that I have seen. Whether Siletn Image can create light or not is going to end up being up to the DM.

* The "reflection" parts are discussed by Crawford during the interview I mentioned. Go find that and listen to it if you want more clarity.
 

gatorized

Explorer
If you can see an illusion because light is reflecting off it, then the spell creates physical matter. The spell can't create physical matter according to the rules.

If you can see an illusion because it's producing light, then it creates light, which is against the rules.

All illusions are invisible.
 

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