A light mountable on a handgun may comprise: a light module including one or more light sources, a cavity for a source of electrical power, a switch, a circuit for energizing the light sources; and an outer housing having complementary parts configured to enclose the light module and a part of the handgun. The outer light housing may include an access cover that is openable when the light is mounted on a handgun, for placing and removing the source of electrical power. Light sources may include an illumination source and an aiming source. A lighting circuit includes a controllable electronic device in series with a power terminal of a controller, wherein operation of the switch causes the controller to maintain the electronic device conductive, whereby the controller then remains powered; and wherein the controller responds to a subsequent operation of the switch to render the electronic device nonconductive, whereby the controller is then unpowered even when electrical power is received.