Determining the preposition before 'which' to replace 'when/where'

Question

When using when/where in adjective clauses, we're allowed to substitute it with this form:

when = preposition + which

Or

where = preposition + which

First of all, I want to show an example:

Monday is the day when they will come then (on that day).

I'm sure it can be rewritten as follows:

Monday is the day on which they will come then.

That example above is what I took from my book and I'm able to understand because there's a hint in the bracket.

Another example I took from a novel: Harry Potter and The Deathly Hallows

He was still at the narrow fragment in which, for a split second, he had thought he saw Dumbledore’s eye.

I want to know where in comes from. At that moment, he's (Harry) holding a broken mirror on his hand, by the way, according to the story. And which conjunction is that? 'Where' or 'when'? My point question is just how to know which preposition we would use, for instance, when we want to subsitute where with preposotion + which.

Answer 1

You don't say what book you took the first example from, but then is incorrect in both cases.

We speak of seeing a reflection in a mirror, so the mirror is the place where we see it. It has nothing to do with time (when).