Light is a very important factor for growing.  Ensuring that our plants have sufficient light, and the correct type of light is key to producing the best crops we can.  The quality of grow lighting used is just as important as the quantity.  The portion of light spectrum that plants are sensitive to is referred to as photosynthetically active radiation or PAR which ranges from about 400-700 nanometers in wavelength.  As seen in the photosynthetic response graph below, plants respond more effectively to red and blue light.  The peak being in the red region around 630 nanometers.  Red light provides the most efficient food for plants.  But, a plant receiving light with only a red/orange spectrum will fail to fully develop. Healthy, vegetative growth requires light from the blue range as well.  The best situation is to have a quality light which is capable of providing all of the light necessary for healthy growth, balanced both in red and blue portions of the light spectrum.  The spectrum of light predominantly emitted from bulbs are generalized with correlated color temperature, measured in Kelvin.  Red bulbs being around 2000 Kelvin, and blue bulbs around 3200 Kelvin and above. 

Plants receiving insufficient light will produce smaller, spindly growth and have an overall lower yield.  While plants receiving too much light can dry up, wither, and become bleached due to the destruction of chlorophyll.  However, if kept within an acceptable range and provided with the correct spectrum, plants will respond very well to light with their rate of growth being proportional to light levels.  In addition to photosynthesis, which is responsible for plant growth, there are other plant actions such as germination and flowering which are triggered by the presence, or absence of light regardless of intensity.  The amount of time the plant receives light is called photoperiod.  Annual flowering plants generally vegetate under longer photoperiods, 18 hours of light or more per day.  While flowering will occur during shorter photoperiods, 12 hours of light per day.  Interruptions or inconsistencies can cause symptoms of stress such as prolonged flowering time, and in some cases intersexual growth traits.