The rules on spellcasting contain the following section:
A Clear Path to the Target
To target something [with a spell], 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.
This section is not really clear to me. Should this mean that only non-transparent objects are a problem in targeting, or that you have to have line of sight and line of effect as well?
Also, can you prevent a wizard from casting spells by blinding her?
Best Answer
A spell does not need line of sight, unless it says so
All spells require an unobstructed path from caster to target.
Spells that say "that you can see" require the caster to be able to see the target, in addition to an unobstructed path.
If the spell does not say "that you can see" then there is no requirement for the caster to see the target. They still need the unobstructed path.
In this episode of the Official D&D Podcast, Jeremy Crawford specifically calls out a closed window (at about 34 mins) - and says it blocks casting.