Can a Minor Illusion block light?

Question

The Minor Illusion spell say:

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

Can the illusory object block light? And if not, what would a creature see?

For example, could a vampire mage use the Minor Illusion cantrip to protect himself from the light of a sun blade? Could a wizard create an illusory box around a torch to "turn off the lights"?

Answer 1

No, it can't.

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"Physical interaction with the image reveals it to be an illusion, because things can pass through it." (PHB, Pg. 260)

When the book references this, it's not just talking about a physical object like a rock. "Things" accounts for all things. This illusory object impedes nothing and is only an illusory trick of the eye, but this is where the concept of magic comes in. Things can pass through the illusory objects even if a creature or player can't register it passing through with their eyeballs. The effects of those things impacting a player would still apply (and perhaps give them a hint as to the object's illusory quality).

There are some great opportunities here for roleplay. I feel like it'd be really interesting to be able to feel the warmth of a torch or fire behind by an illusory wall. I feel like a wizard casting Minor Illusion to block the light of a campfire from an enemy patrol is an engaging and creative use of the spell.

I would ultimately say that it's up to you to decide where to draw the line, but that the rules state this is nothing but an illusion and doesn't have an effect on any properties of the world around it.

Answer 2

Can the illusory object block light? And if not, what would a creature see? ...Could a wizard create an illusory box around a torch to "turn off the lights"?

The spell creates an image. If the image can't block light, then it would be transparent, or at least translucent, which would severely limit the utility to the point of near uselessness. Certainly an object that light passes through when it shouldn't normally, such as a stone wall, would raise suspicion and give the illusion away. So we have to posit that the image can block your view of things behind it, including light sources, if you're fooled.

The spell description says (PHB, p. 260):

"If a creature discerns the illusion for what it is, the illusion becomes faint to the creature."

implying that the image appears solid otherwise. Creatures who haven't figured out that it is an illusion should see the image as though it were the intended object, with all of its requisite visual properties. So I would expect that someone fooled by the spell would be tricked by the magic into thinking the light was blocked and would "see" the environment in whatever reduced lighting conditions this caused.

I also expect there will be clues presented in such a scenario that warrant allowing creatures to make an investigation check. If the object suddenly appears, or a light is suddenly dowsed, a creature would be likely to investigate the event more closely, especially given that their senses are actually contesting the magic whether they fully realize it or not.

For example, could a vampire mage use the Minor Illusion cantrip to protect himself from the light of a sun blade?

As for a vampire protecting themselves from a source of sunlight using this cantrip, clearly the vampire "...discerns the illusion for what it is..." which should negate its effectiveness in shielding him from the light source.

Indeed, I would probably rule that even if a vampire believed the illusion was shading them (say someone else cast it for them), it's actually not, so they'd take damage and probably figure out right away that something was not as it seemed. I consider this to be the same case as trying to shield someone from a ray-based spell attack using this cantrip. The ray isn't fooled, and neither is your body when the ray hits you.

That said, the spell could block someone's line of sight, and if they couldn't see through it, they may not be able to see their intended target. So you could use the spell to block an opening, like an arrow slit, to potentially prevent yourself from being targeted by a wizard casting through the slit.

Answer 3

RAW, it's unclear. But I'd rule that it doesn't block light, except that it seems to for the viewer.

If you were in a windowless room with one torch, and you created the illusion of a box around the torch, people couldn't directly see the brightness of the torch, but they could see the fact that the room was still lit by torchlight, and even (with a suitable investigation check) figure out where the torch was by looking at shadows.

This means that it could prevent some effects of the light, such as blinding the viewer (if the viewer hadn't seen through the illusion); but not others, such as any radiant damage it dealt.

An illusory umbrella would be useless to a vampire or a way-of-the-shadow monk. It might help a drow, but only in very borderline cases where the ambient light was not bright but the direct light was.