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Some spells only require line of sight for targeting. Can you use a mirror to look around a corner and target your foe?
For example, Magic Missile:
The missile strikes unerringly, even if the target is in melee combat or has less than total cover or total concealment.
According to d20srd:
Line of Effect A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It’s like line of sight for ranged weapons, except that it’s not blocked by fog, darkness, and other factors that limit normal sight.
You must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect. You must have a clear line of effect to the point of origin of any spell you cast.
All of the other entries just say "line of sight" without referencing calculations. The term "straight, unblocked path" suggests that a mirror would not provide a straight path. However, this is sufficiently murky that beyond this citation there is nothing suggesting anything either way and should be referred to the GM.
I believe the use of a mirror should allow for establishing line of sight. However, line of effect is more the issue here. Most any attacking power requires line of effect to the target, or (for burst/blast effects) requires line of effect to an origin square from which there is line of effect to the target.
Especially, consider the verbiage noted in Magic Missile as you've cited:
The missile strikes unerringly, even if the target is in melee combat or has less than total cover or total concealment.
(Emphasis mine.)
In most cases where you would want a mirror to establish line of sight, the target you're working against would have total cover from you. Since the mirror only helps you get around concealment, the total cover would render Magic Missile unusable.
Given that the rules are unclear on the subject, a good GM call might be to say "if the spell effect could be reasonably considered to bounce off of mirrors then yes, otherwise no".
I would say no, Line of Sight is really just a shortcut for saying "an unobstructed clear path between you and the target". That said, the target might be in the line of sight, but have cover. Anyway, around corners ... nope, certainly not the same thing, IMHO.
I would say that there should be a concealment miss chance, similar to trying to avert ones gaze against a gaze attack. So there is a 20% miss chance. If used, say, against a medusa, then there would be a 50% chance to avoid staring at her, and thus having to avoid making a Fortitude save.
No,
Mirrors do not provide an exception to the need to have a clear Line of Sight.
Nor would any other form of magic allow you to get around it. (Yes, Mirrors are magic, didn't you know that? :))
It's not RAW, but I will note that a 1 foot square hole in a wall gives spells line of effect.
"An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell's line of effect."
So you could rule that a mirror of at least 1 foot square could give you line of sight. That's a good compromise - no peeking through keyholes or using reflections of buttons or shiny silver pieces weaseled around a wall.
I'd assume not. Think of Perseus, (though Perseus isn't a dnd campaign, I think the same logic applies) that it is not a direct line of sight.
