California Code, Penal Code - PEN § 653n

Any person who installs or who maintains after April 1, 1970, any two-way mirror permitting observation of any restroom, toilet, bathroom, washroom, shower, locker room, fitting room, motel room, or hotel room, is guilty of a misdemeanor.

This section does not apply to such areas (a) in state or local public penal, correctional, custodial, or medical institutions which are used by, or for the treatment of, persons who are committed or voluntarily confined to such institutions or voluntarily receive treatment therein;  (b) in private custodial or medical institutions, which are used by, or for the treatment of, persons who are committed or voluntarily confined to such institutions or voluntarily receive treatment therein;  (c) in public or private treatment facilities which are used by, or for the treatment of, persons who are committed or voluntarily confined to such facilities or voluntarily receive treatment therein;  (d) in buildings operated by state or local law enforcement agencies;  or (e) in public or private educational institutions.

“Two-way mirror” as used in this section means a mirror or other surface which permits any person on one side thereof to see through it under certain conditions of lighting, while any person on the other side thereof or other surface at that time can see only the usual mirror or other surface reflection.