There are no rules to my knowledge saying light pass through obstacle like total cover wall, glass, nor there are rules on glass indicating one way or another.
DMs can make ruling that glass is or isn't total cover.
DMs can make ruling that light may or may not emit through glasses.
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.
How is that RAW? An AOE extends until blocked. But in which rule does it say a pane of glass blocks light? That's explicitly what it doesn't block.
The thing is all the AoE spells should follow the same rules, so for example if the area of effect of a Fireball is blocked by obstructions that provide total cover, such as dungeon walls or a wall of glass, the same should apply for other AoE spells like Light, Darkness, Silence or Spirit Guardians.
But, if you're asking for my opinion, I've already mentioned how I rule Light and the light created by that spell in my games in a previous post:
However... in this specific scenario, I was pointing out that light is light. So, if we're talking about a regular transparent piece of glass like those in the real world, the light should pass through it.
Non-magical light passes through glass as expected. But if the light is the result of a magic spell's AoE, then it's just another magical effect and those are blocked by physical barriers as per the rule for an AoE.
Can you quote the rule excerpt that says that? Because I can't find it.
I can only find it saying that barriers can block the effects of the aoe. But since glass doesn't block light it wouldn't be a barrier to the effect of the light spell.
This has been mentioned earlier in the thread, but just to answer the question and to generally recap the idea -- this is the rule which dictates the area that an AoE spell effect can occupy:
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.
No exceptions are made within this general rule about which particular spell effect it is. For a specific vs general exception to be made, it would have to be explicitly stated within the spell description.
Note that this relies on the ruling that the particular window of glass in question provides total cover, but that seems to be the most common ruling within the community.
Sure but total cover affects the targeting of spells, it has no stated effect on saving throws. Don't get me wrong, I know that it is a popular interpretation of the spell and that Crawford has said multiple times that Sacred Flame works even against a target that has total cover. If that is their intention it would be helpful if they made an errata to the spell.
The rule for total cover doesn't make any reference to providing any benefits to Dexterity saving throws because spells cannot target anything that's behind total cover in the first place.
But Sacred Flame doesn't say the target gains no benefit from cover to his saving throw. It says the target gains no benefit from cover for this saving throw.
So for this saving throw, the target gains no benefit from cover as described in half cover, three-quarters cover and total cover and which include among other things that you can't be targeted directly by an attack or a spell.
At least that is the popular interpretation as you said, shared by Devs and to this date received no errata to better clarify wording.
While reading the new 2024 spells, I remembered this answer from you. It seems now Sacred Flame has a better wording:
Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 Radiant damage. The target gains no benefit from Half Cover or Three-Quarters Cover for this save.
Technically, total cover is provided as long as there is a physical barrier regardless of whether you can see through it and most spells are blocked by total cover.
This is one of those "the devs didn't think this through" things. They kinda forgot glass existed. So, RAW is kinda stupid here. The rules are based around typical dungeon and blasty spells. Non-physical spells being blocked by any object - even PAPER - is nonsensical.
Normal common sense logic would have your bolt of lightning or whatever be able to blast through 'fragile cover' and still hit the target, anything physical or with enough oomph should go through the glass. A window would, according to the DMG, have about 3 hit points and AC of 13. If the spell ignored AC or the attack roll was greater than 13 I'd let the spell go through, breaking the glass, and deduct the 3 hp from the spell's damage. I'd also ignore the total cover rule for spells that have no physical substance like minor illusion or whatever.
A few spells, mostly teleportation stuff and Sacred Flame (which specifically states it ignores cover), there's also Detect Thoughts and Raum's Psychic Lance, which can ignore cover too.
I actually can't remember the last time a game has glass windows or anything like that. Potion bottles are in glass but I think that is it.
We normally played Sacred Flame as an effect centered on a spot but descending from the ceiling or air/sky above the target. Thus they would have to have overhead cover to be protected.
But what about spells with the 'you can see'? Can you cast Find Familiar through a glass window?
There are no rules to my knowledge saying light pass through obstacle like total cover wall, glass, nor there are rules on glass indicating one way or another.
DMs can make ruling that glass is or isn't total cover.
DMs can make ruling that light may or may not emit through glasses.
The thing is all the AoE spells should follow the same rules, so for example if the area of effect of a Fireball is blocked by obstructions that provide total cover, such as dungeon walls or a wall of glass, the same should apply for other AoE spells like Light, Darkness, Silence or Spirit Guardians.
But, if you're asking for my opinion, I've already mentioned how I rule Light and the light created by that spell in my games in a previous post:
This has been mentioned earlier in the thread, but just to answer the question and to generally recap the idea -- this is the rule which dictates the area that an AoE spell effect can occupy:
No exceptions are made within this general rule about which particular spell effect it is. For a specific vs general exception to be made, it would have to be explicitly stated within the spell description.
Note that this relies on the ruling that the particular window of glass in question provides total cover, but that seems to be the most common ruling within the community.
While reading the new 2024 spells, I remembered this answer from you. It seems now Sacred Flame has a better wording:
I actually can't remember the last time a game has glass windows or anything like that. Potion bottles are in glass but I think that is it.
We normally played Sacred Flame as an effect centered on a spot but descending from the ceiling or air/sky above the target. Thus they would have to have overhead cover to be protected.
But what about spells with the 'you can see'? Can you cast Find Familiar through a glass window?