Can a spell be cast through (semi) transparent things?

Question

Abby, the little gnome illusionist, ran to the nearest window. Grabbing the windowsill, she stood up on tiptoe, getting her chin on it. Unfortunately, the rough, tinted glass hinders what could be seen through it, so Abby glanced at the window latch: too high for her. In any case, she could clearly identify, looking through the window, two humanoids on the outside. And their rough language indicated that their intentions were unfriendly.

— I must quickly draw them here so they can not do any harm! —

Lots of spell came to her mind: 'Fire Bolt', 'Sleep', 'Ray of Enfeeblement' or even 'Silent Image' but Which of them, if any, could be thrown through the glass?.

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Note: The answer of this question, and some tweets could help to clarify the question.

Update: this question asks the meaning of "spot" in a spell description when casting that spell.

Answer 1

Generally, no. But it depends on the spell

Fire bolt, sleep, and ray of enfeeblement cannot pass

The three spells all require you to target something. The things you can target are all bound by the rules on how to target things, and one of those rules is you cannot target someone behind total cover.

PHB pg 204, A Clear Path to the Target

To target something, you must have a clear path to it, so it can't be behind total cover.

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction

"Total cover" is defined in the PHB, so this is a game term.

PHB 196, Cover

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

Ask if a different character in the same position could have targeted the two humanoids with a bow. If they could not be targeted with a bow (or some other ranged attack) due to an obstruction, then they are behind total cover. In your example, this is the case (arrows normally do not phase through translucent glass).

The targeting rules apply to all spells: those with attack rolls, saving throws, those that target objects, creatures, or points of origins (to create areas of effect). Unless the spell description overrides these rules or comes into conflict with them, any spell follows the rules on targeting objects and requires a clear path to the target.

Silent Image can pass through the glass

Silent Image has a range of 60 ft and says the following:

The image appears at a spot within range and lasts for the duration.

This explicitly overrides the targeting rules, in that the image may explicitly appear at any spot within range. This includes spots within range behind total cover.

The spell also says the following:

You can use your action to cause the image to move to any spot within range.

So if you conjured the image somewhere not behind total cover with respect to you, you can still use your action to move the image somewhere behind total cover anyway, as long as it doesn't go beyond the range of the spell.

Answer 2

Jeremy Crawford confirmed that spells will not go through transparent obstacles.

In a podcast, he confirmed that you can't target enemies with spells beyond transparent obstacles because spells require a connection between caster and target.

Regarding spells where you can choose the point of origin, unless they specifically mention they can be cast on places with total cover, they won't work. This is due to the 'specific beats general' guidelines. Spells like Fireball (where you pick a point of origin) and targeted spells (that you cast without realizing there was an obstacle between the both of you), would both explode just before the obstacle, since

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction

This agrees with the tweet you posted, where Wall of Force (transparent) still block attacks, since

in general, a barrier that stops physical objects stops spells

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Keep in mind that there is still a distinction between cover and concealment:

Cover is a physical obstruction, not necessarily a visual one.

Jeremy also commented on how Conjure X are affected by this:

Unless a spell says otherwise, you can't cast it at someone or something behind total cover.

Conjure Fey, for instance, says:

You summon a fey creature. It appears in an unoccupied space that you can see within range.

In other words, spells with the "unoccupied space" terminology also do not work over total cover.

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This seems to imply that spells like Misty Step also require LoS.

You Teleport up to 30 feet to an unoccupied space that you can see.

However, remember that Misty Step has a range of Self. You are not casting it on some other point (which would require a clear path between you), but instead are casting it on yourself, and appearing in a point you can see (even if a big glass window is between you). Teleports like Dimension Door have a range of 500ft, but specify any other spot within range.

Answer 3

It's clear, as per the rules, that you can't aim at a target that you can't see (it has total cover). It makes sense. The problem arises when, as your question poses, you can see it but there's a transparent obstacle between both. @Markovchain and @Bluemoon93 have opposing views on the issue.

It is the opinion of one of the game creators that magic "projectiles" - spells that travel following a path from the caster to the target - "explode" whenever they hit anything solid: even a glass would stop a fireball. The fireball would destroy the glass instead of hitting its intended destination.