Light intensity is defined as the concentration of light waves striking the plant's leaf surfaces. For optimal growth and flowering, plants require the proper intensity and duration of light. This is true whether you are using sunlight, artificial lighting, or a combination of both. Usually, light intensity and duration are the limiting factors when maintaining plants indoors. The primary reason plants need light is photosynthesis.
Photosynthesis is the conversion of light energy to chemical energy by plants.
All life depends on this process. Plants need to photosynthesize to exist, and animals need plants for food as well as oxygen, which is a by-product of photosynthesis. Light also affects plants in other ways. Phototropism, the bending of plants toward light, will greatly affect plant form. Low-intensity light, with more red rays, will cause tall, spindly plants.