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If I cast light on a small pebble and placed it in a sealed, transparent glass jar would it emit light outside of the jar?

The reason I am asking is because the rules for areas of effects say:

A spell’s effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover.

That means that the jar (which provides total cover) would prevent anything outside of the jar from being in light's area of effect. So, it seems to me that the rules imply that nothing outside the jar would be illuminated.

This actually does have potential mechanical implications in the game as can be seen in this question: Can magical darkness affect a magical light source that is in a sealed container?.

Am I missing something here or is this just an example of the rules creating a nonsensical effect?

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Because we are exploring line of effect, it is important to note what, exactly, the effect is. If the effect of light, for example, is to create light centered on the object, the transparent container blocks that light since it blocks line of effect via full cover. If, on the other hand, it makes the object glow brightly enough to emit that light, the effect is constrained on the object itself and the light is a desirable side effect, kind of like setting things on fire with a fireball. In that case, the light wouldn't be blocked by glass any more than torchlight would. The key to distinguishing this in the spell is the word "sheds."

You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet.

I've checked the PHB. The only objects that shed bright light are also the sources of that light, whether it be the flame from produce flame or that of a candle. Light, therefore, makes an object glow and the light it sheds will go right through glass.

Darkness makes a similar distinction:

If the point you choose is on an object you are holding or one that isn’t being worn or carried, the darkness emanates from the object and moves with it.

If you cast darkness on an object, the magical darkness is being emitted and will pass through glass, as counterintuitive as it sounds. If you cast the spell on an area, it would be blocked by glass.

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RAW, probably not. RAI... probably?

Jeremy Crawford has stated that:

A solid obstacle, regardless of material, can provide total cover. A closed window counts.

Whether a jar counts as an "obstacle" is largely a matter of opinion, but it is difficult to create a definition of "obstacle" that applies to a window but not a jar. As such, RAW likely prevents the light from penetrating a glass jar.

However, there is a clue to the RAI in the text of the Light spell, which states:

Completely covering the object [which is the point of origin of this spell] with something opaque blocks the light. [PHB, p. 255, emphasis added]

It is peculiar that the spell specifies that an opaque object is necessary, as by a strict RAW reading, covering the point of origin of this spell with anything (transparent or opaque) would block its effects. Although this does not give us a solid RAW reason to conclude that the Light spell would penetrate glass, it does give enough logical wiggle room for a DM to interpret the light created by the spell as penetrating the glass.

A word of warning: however you interpret this ruling, it should logically be similarly applied to the Darkness spell, which also specifies that it is blocked by covering its point of origin by an opaque object. So if light from the Light spell would go through a glass jar, so too would darkness from the Darkness spell.

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Light Spells

Light

You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light...

It then describes how bright that light is.

Said object then sheds light with all of its glorious perks and limitations including going through transparent material. This effect is not described in such a way that leads me to conclude it is an emanation/sphere effect.

The same holds true for Continual Flame:

A flame, equivalent in brightness to a torch, springs forth from an object that you touch.

These spells simply make something else shed light.

Now things like Sickening Radiance are a different story:

Dim, greenish light spreads within a 30—foot-radius sphere centered on a point you choose within range.

It seems word choice is improving as they go through iterations of source material and this one is fairly clear cut.

Darkness

In comparison:

Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration.

This effect is definitely a sphere (what used to be called an emanation) effect.

It gets muddy when you look at the two when they interact.

Again from Darkness:

If any of this spell’s area overlaps with an area of light created by a spell of 2nd level or lower, the spell that created the light is dispelled.

The effects of Darkness not only create a globe of magical darkness but also dispels any source of magical emitted light it touches. This seems to point toward Light being an area of effect spell even though, by its description, it is clearly not.

As TheVagrantDog explicitly calls out even if you put a magical light within a tranparent sealed container it would still be dispelled by Darkness because it specifically says it does not have to touch the source of the light just any light shed by it.

This choice of wording seems to be made poorly, as several interactions are, unfortunately.

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    \$\begingroup\$ Good point on the dispelling. That means that even if you rule that line of effect protects the light source from interacting with darkness, if light was being shed through the glass the spell would still wink out. \$\endgroup\$ – TheVagrantDog Apr 25 '18 at 17:24
  • \$\begingroup\$ @TheVagrantDog Indeed, I would liken it to a feedback loop which disrupts the existing magic. \$\endgroup\$ – Slagmoth Apr 25 '18 at 17:40
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In short, a transparent barrier would not block either. From Light:

... the object sheds bright light ...

From Darkness:

Magic darkness spreads from a point you choose within range ... The darkness spreads around corners.

So, in the case of Light, the object glows, shedding non-magic light just as any other glowing object. Therefore, a transparent barrier would not impede it, much as if you had a strong enough transparent container and cast a fireball in it (from the inside, presumably), the barrier would stop the fireballs effect (fire damage), but the fireball would still be observable outside the barrier from the emanated light.

However, with Darkness, the darkness is considered magical, and thus bound by normal line of effect mechanics. It even goes around corners in a way that normal light would not. However, the text of the spell specifically states that you must cover the source with an opaque object, thus implying that a transparent object would be insufficient, and therefore overriding the generic rule with a specific rule.

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The description of the light spell reads:

Completely covering the object with something opaque blocks the light.

Also, the description of the darkness spell reads:

Completely covering the source of the darkness with an opaque object, such as a bowl or a helm, blocks the darkness.

The specification of opaque, implies that the light/dark is an emission emanating from the source.Thus for spells involving light/darkness line of sight is the key element. Thus, the area of effect is determined by line of sight. This could be expanded to include all effects involving vision.

Transparent objects don't block line of sight. Let's take this a step further. If you rule that light is blocked by the transparent jar. Then similarly, I could wear a sheet of glass on the front of my helmet to block all gaze attacks.

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  • \$\begingroup\$ Line of sight is not the issue here. It is obvious that if I put a torch in a transparent container that its light would be visible. Line of effect (though technically not a thing in 5e by that name) is what I am interested in exploring. \$\endgroup\$ – Rubiksmoose Apr 25 '18 at 16:34
  • \$\begingroup\$ If that is your intent (and I think that logic is definitely worth exploring), I would put that into your answer in more detail. As it is, your answer doesn't really answer my question right now. \$\endgroup\$ – Rubiksmoose Apr 25 '18 at 16:39
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    \$\begingroup\$ transparent objects d0 block line of sight. \$\endgroup\$ – NautArch Apr 25 '18 at 16:55
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    \$\begingroup\$ @NautArch do you mean "line of effect"? Line of sight should still be there since you can see through it right? ravery, I have not downvoted but I am not yet convinced that line of sight is relevant here really. Just because an opaque object blocks the light, does not mean that a transparent one does not. \$\endgroup\$ – Rubiksmoose Apr 25 '18 at 16:59
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    \$\begingroup\$ @NautArch - and specific overrides general. Both light and darkness spells specify that the object must be opaque to block it \$\endgroup\$ – ravery Apr 25 '18 at 17:10
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Yes.
It will emit light outside the jar.

"... Until the spell ends, the object sheds bright light"

This is an oil lamp but with a pebble instead of a burning wick.
It sheds light.
It does not create a magical effect that lights up an area by some weird excitation of molecules spreading from the target. The spell's effect is to make the target glow - to shed light.

Really, this question is "does light go through glass" and if you take out the nearest window I think you'll find it does. So long as that glass is transparent.

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To prevent the rules from creating a nonsensical effect, I think it's important to keep the magical and nonmagical effects separate.

The rules about line of effect and area of effect describe how magic works. To hit someone with a fireball, you need line of effect between yourself and the target.

I don't think line of effect applies with a Light spell, because it has a range of "touch". You touch the coin, infuse it with magic, and it starts glowing. Now the coin has magic in it that's producing light.

Once the light leaves the surface of the coin, it's just light, and light passes through glass.

Now, let's say that you take your glass-encased penny into a Darkness spell. As soon as the light reaches the other side of the glass, it is blocked/absorbed by magical darkness--but the glass provides the coin with total cover against the Darkness spell, protecting it from being extinguished. When you bring your penny back out of the Darkness, it will still be glowing.

TL;DR Glass blocks magic, but it doesn't block light.

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