Like Cyrad said, mirrors tend to reflect light (though that's not saying that certain spells can't be made that can be reflected). If I had to Houserule something up on the fly, the feat would probably go along the lines of:
Mirror Bounce:
Prerequisites ability to cast any three ray spells, Weapon Focus (ray)
Description
'You can bounce your ray spells off mirrors or other highly-reflective surfaces. You make your attack against the mirrored surface (size modifier to AC applies) and then an attack against the target or another mirrored surface with a cumulative -2 attack penalty for each mirror already struck. Additional penalties may be applied for bouncing off damaged, warped, or poor-quality reflective surfaces, such as a cracked or dirty mirror.
Ray spells whose effects are entirely or very nearly-entirely composed of light, even those without the [light] descriptor (like searing light), can travel up to their full range without losing potency to their effect, damage, or save DCs. Other damaging rays deal half their damage to each surface struck (which might be a creature in highly-reflective armor), losing potency with each redirect. For instance, a ray spell that would deal 30 damage, deals 15 to the first mirror struck and 15 to the target, or 15 to the first mirror, 7 to the second mirror, and 7 to the target. Rays that deal no damage instead lower their save DC by 2 for each surface deflected from.
Mirrors or surfaces destroyed by damage or effects from the ray (such as disintegrate) do not prevent the ray from continuing on, though a surface or target can never be struck or affected more than once by a single ray spell. Casting a ray spell using this feat always takes at least a full-round action (unless the spell normally has a longer casting time) as that time is spent aiming and lining up the attack.'
That's just a quick a dirty suggestion of course. I haven't looked over all ray spells and how that can be abused. Obviously a caster of disintegrate could wreak havoc on a room of highly-reflective objects assuming he could hit each one with the stacking attack penalties. So might have to have a provision in there for not allowing secondary effects on 'bounced' targets (like just disintegrating objects straight-out without dealing enough damage), but I don't want to get too technical or exacting for a quick example. I guess if you counted the green beam as strictly light... then it would still only effect its final target, which would be fine with the attack penalties... not sure I consider it in the 'light' category though.
