The biggest role that light plays in regards to a growing plant is its help in triggering photosynthesis. Photosynthesis is the process by which growing plants produce food. Light that is absorbed through the plant walls is a key player in photosynthesis. The more sunlight a plant receives, the more food it can produce. Photosynthesis is key to plant development; without it, plants would not have the energy to grow.
Light also encourages growth in developing plants. The wavelengths that make up the light prism come in a variety of colors. The red and blue wavelengths affect plant growth the most. Blue light helps plants' leaves grow, while red light can trigger flowering. These wavelengths are present in sunlight, but are often not found in indoor lighting from bulbs. When plants are grown indoors, you may notice that they develop much more slowly than outdoor plants for this very reason. Sometimes, grow lights that emit a mixture of red and blue wavelengths are used to compensate for this deficiency.
Flower development is also related to light exposure in growing plants. Different plants require varying lengths of photoperiods. A photoperiod is the amount of continuous time that a plant spends in darkness each day. Some plants are extremely sensitive to light, and their exposure must be limited until they bloom. Others require very short photoperiods to flower, meaning that they thrive during long, sunny summer days. On the flip side, these plants sometimes require the aid of a grow light to bloom if the days are not quite long enough for them.