Question about darkness, faerie fire, and blinded (re: (dis)advantage)

People are busy making the error of the converse. "Opaque objects block darkness" does not mean "non-opaque objects do not block darkness", and in fact opaque things that do not count as a barrier and thus do not provide total cover (such as a cloth) can block the spell.

What you can say about darkness is "the developers don't understand standard terminology for vision". Seriously, anyone who understood what they were doing would have solved this problem by making darkness and heavily obscured different effects. Since they have not done so, we have no way of telling what the darkness spell actually does.

Quote from MaximusArael020 >>

Quote from Squirrelloid >>

Quote from up2ng >>

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[...] You said "When was the last time you saw normal darkness radiate through a glass pane?  Because the Darkness spell does that." Um.... the Darkness spell cannot radiate through a glass plane, technically. At least, not if you cast it on an object. Because when you cast in on an object, it becomes an Emanation. An Emanation states: "An Emanation is an area of effect that extends in straight lines from a creature or an object in all directions. The effect that creates an Emanation specifies the distance it extends."

When we look at the "area of effect" rules we see: "If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover. See also “Cover.”" [...] 

 (sorry for snipping your post. I just wanted to address this part)

I agree with you that the Darkness spell can be blocked by a glass pane. I also remember a thread (Rules Clarifications) in which up2ng and Squirrelloid were discussing this interaction, along with some other spells (Wall of Force, Antimagic Field). Here's one of my replies from that thread:

Quote from Tarod_ >>

Quote from Squirrelloid >>

[...] (And if glass is an obstacle, then it blocks the AoE of a light spell, which makes no sense.  Glass is obviously not an obstacle to light). [...]

 Obviously, you don't need to agree with me, but this is my interpretation from when your question was discussed in another thread (What spells can you cast through glass?), based on the rules ("If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect"):

Quote from TarodNet >>

The next image is just to illustrate my point of view and to receive your feedback (if you want to join this amazing party!)

Note: the ranges are simplified. Instead of 20 ft, it's a 10 ft radius for both bright and dim light. The inner circle represents bright light, while the outer circle represents dim light.

RAW, the pane of glass blocks the AoE of the light spell, so its right side won't be affected by the spell, thus limiting its AoE.

Common sense tells me that once the light is created, it should reach the other side of the glass pane, perhaps not fully, depending on the type of glass and other factors, which would be up to the DM to decide. However, this won't be due to the AoE itself, but because it's light in this specific case.

But it's true, strictly RAW, you could argue the light, being magical, cannot pass through the object even if it's transparent. So, for example, if the right side of the glass were natural darkness, that side would remain dark. 

Regarding Antimagic Field (also discussed in Rules Clarifications, btw), I don't think it provides Total Cover for the purpose of targeting. But this is a funny spell (*), so for entertainment's sake: Does Antimagic Aura affect the caster?

(*) as the Dev once said:

@koboldskeep Do you wish you had kept 3e's Ex/Su/Sp ability descriptors?
@JeremyECrawford Nope. My only regret in this regard is that we left the open-ended "magical effects" wording in some places. Also, antimagic field is a funny spell in that it's the star of many internet discussions, despite not seeing frequent use at the game table.

And I still don't agree.  Obstacle-ness is assessed on a case-by-case basis.  The obstacle must actually obstruct the thing in question, otherwise it's not an obstacle.  And if it's not an obstacle to the thing, it cannot contribute to Cover against it, because Cover requires obstacles.  It makes no sense to say something is an obstacle which does not impede the thing in question. 

How do you determine what's an obstacle to various things?  DM adjudication, but should follow basic logic.  The rules provide no guidance on how we determine what an obstacle is.  This is where we use reality or logic to decide.  Light is light, we know it shines through glass, so glass is not an obstacle to light.  Psychic effects are psychic, they ignore physical things, because they aren't physical by definition, and cannot interact with physical things. 

(Non-opaque) Glass is not an obstacle to light, so cannot not provide cover against it because being an obstacle is required to contribute to cover.   And Darkness is described in the same way as the Light spell with regards to what obstructs it, so glass is also not an obstacle to it.  And magical light is still light.   

Quote from Squirrelloid >>

And I still don't agree.  Obstacle-ness is assessed on a case-by-case basis.

 D&D treats cover as a binary property.