Mirrors are used as portals in fantasy because it seems to make sense psychologically, to people who are pre-scientific.
Now we understand that reflective surfaces reflect images if sufficiently smooth, and a common mirror reflects light because metal is a good conductor, and this reacts to the propagating electric field. Other smooth surfaces, like calm water, reflect partially for different reasons.
Knowing what we do now about mirrors, how can we explain why magic portals are associated with them?
I want the same (real) reasons objects function as good mirrors to be involved in why the portal spell operates on that surface.
If casting a spell for remote viewing or transport or opening a demonic realm, I need a flat surface to enchant. But surfaces that are especially suited happen to function as mirrors normally. In the old days nobody wondered as they figured mirror viewing was related magic. But now we know that it’s certain physical properties of the surface that make it work as a mirror.
How is the magic related to these properties?