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?

So, basically you are asking why smooth, conductive surfaces would be good catalysts for magic?

First I should mention that the surface of modern mirrors is made of glass, which has poor conductivity. Keep that in mind when designing your mirrors. Perhaps you could use this to explain why some mirrors are useful, while some aren't.

As for the answer, if magic were to be special subtype of electricity, that would explain the benefit of conductivity. The smoothness of the surface would then also be beneficial, as rough surfaces don't conduct electricity as well when making contact.

It could have something to do with the fact that it's an area that doesn't corrupt the light that hits it, or something like that... perhaps the portal's created by tweaking the method by which the mirror manipulates the light, to do something beyond my understanding, which causes a portal to form. For this to work, I imagine the light would have to be pure both before and after reflection, and a mirror is the obvious choice for this.

You could have it being possible to create portals on less reflective surfaces, like ponds etc, but have them be more unstable, or only able to be used for viewing, not transporting, etc.

Perhaps it's not so much that they are mirrors, but rather what they are capable of reflecting. The eyes are considered to be the window of the soul, perhaps the mere ability to look directly into one's soul is power enough.

As an interesting twist, there may be some spells that call for you to gaze into your own eyes for the spell to work. Other spells might forbid it, for fear that they might consume you if you saw yourself. It also makes slightly rippling ponds and fun house mirrors into rather interesting topics.

A mirror creates topology. In a mirror that is just a mirror you see your own world reflected in it, but if you look behind the mirror you see nothing. So visually the mirror creates a bounded multiply connected topology.

Couple this with an identical mirror. Each when a mirror creates their local topology. Connect them and the local topologies reverse. Now you are in effect behind the other mirror in terms of what you see.

For it to work as a portal, the mirror is either destroyed on passage, or has to be made of a material that is self healing. Or it takes place on the interface between air and mirror surface. For these reasons a back surface glass mirror cannot be used as a portal. But a front surface mirror can be. Silver on glass works. A back surface mirror can still be used for viewing, but it is vulnerable to a projectile coming through the mirror, which would shatter the glass.

Any part that doesn't reflect however is in effect solid. So if there is scratch on the surface of the mirror it would act like a wire and cut you in half. Point like holes act like tiny needles. Since perfect surfaces are hard to maintain, traversing a portal would likely be very painful.

To counter this you would put a protective spell on the surface, and maintenance mages would have spells to reveal defects in the mirror surface. Portals used regularly would be maintained at both ends. A secret mirror on the other end may not be maintained. First person through may be injured or killed.

One of the subterfuges of war would be to smuggle in a portal mirror.

Even a small mirror would be sufficient. E.g. Start with a 3" mirror. Once esconced in the castle, your other end passes through a 2.5" x 12" oval mirror. Then through that you can pass a 10" by 4 foot mirror Then through that a 2 x 6 foot mirror. Then you can send a couple of helpers through to handle your end of a 5 x room height mirror that can take a mounted rider.

This can allow portals to act as industrial devices. Score a portal with fine lines, and turn rocks into sand. It would be the ultimate vegematic.