Oofta
Legend
I was prompted to raise this topic when after watching a play stream where one of the characters was glued to the front of a vehicle and, as the character's head then couldn't turn sufficiently, there was no ability to cast healing word on passengers behind.
If a mirror was used to see the passengers, would the "A Clear Path to the Target" requirement be satisfied?
Does the caster have line of effect? My test that question is simple: if they were shooting an arrow in a straight line could they have hit the target without breaking something in between*? Next, in the case of healing word it has the additional limitation of "that you can see".
I'd allow "that you can see" to be covered by the mirror. The windshield, if there is one, on the other hand could block the line of effect.
*That can get a bit tricky, in general though the arrow rule still applies. An arrow would penetrate, say a sheet you were hiding under if you were silly enough to think that would work. If you were totally concealed behind a tower shield it would not.