A light source includes a controller configured to turn on or off a plurality of light sources depending on a light period. The controller can be configured to turn on the light sources during each of a plurality of light periods such that the light sources emit a light having a spectrum with a plurality of peaks toward the plant. At least one light period can include a first period and a second period and the first period preceding or following the second period. The controller can adjust the spectrum of the light between the first period and the second period and/or during different light periods.