Lighting a mirror is the one lighting job where the fixture is not the point — your face is. Get it wrong and every shadow lands in the wrong place. Get it right and you stop noticing the light at all.
The shadow problem
A single light directly above a mirror casts shadows downward, straight into your eye sockets and under your chin. It is the worst possible arrangement for shaving or applying makeup, and it is also the most common.
The fix is light from both sides. Two fixtures either side of the mirror at eye level cross their beams on your face and cancel each other's shadows. If the wall has no room for two, a wide bar above the mirror is second best — spread beats a single point.
Getting it right
Side fixtures — mount at 60 to 66 inches from the floor, roughly 28 to 36 inches apart.
Above the mirror — the fixture should be about three quarters of the mirror's width, mounted 3 to 6 inches above it.
Colour temperature — 3000K to 4000K. Warmer than that and you cannot judge colour; cooler and the room feels like a hospital.
CRI 90 or higher. Below that, skin tones read wrong and makeup applied at the mirror looks different in daylight.
Backlit mirrors
An LED strip behind the mirror glass gives a halo of light on the wall. It looks excellent, but it is ambient light, not task light. Treat it as an addition to side fixtures rather than a replacement for them.