BICYCLE LIGHTS LAW REQUIRMENTS IN Pennsylvania
In US the bike lights die required by Law if you reading in low light conditions, The light can be attached to either the bike or to the cyclist but must be visible at least 500 feet to the front of the bicyde, so be careful how you position the light. Additional lighting must be consistent with the law specifying required lights and reflectors, so a rear light must be red.
75 Pa. Cons. Stat. §§3507 (2016)
§ 3507. Lamps and other equipment on pedalcycles.
(a) Lamps and reflectors.--Every pedalcycle when in use between sunset and sunrise shall be equipped on the front with a lamp which emits a beam of white light intended to illuminate the pedalcycle operator's path and visible from a distance of at least 500 feet to the front, a red reflector facing to the rear which shall be visible at least 500 feet to the rear and an amber reflector on each side. Operators of pedalcycles may supplement the required front lamp with a white flashing lamp, light-emitting diode or similar device to enhance their visibility to other traffic and with a lamp emitting a red flashing lamp, light-emitting diode or similar device visible from a distance of 500 feet to the rear. A lamp or lamps worn by the operator of a pedalcycle shall comply with the requirements of this subsection if the lamp or lamps can be seen at the distances specified.