What the light should be like and why do plants need them?
Plants use light energy in the photosynthesis process, which, in a nutshell, converts the light energy into chemical energy, consumed by the plants for growth or fruiting. This process is made possible by two types of chlorophyll, present in plants: A and B. The chart below shows that chlorophyll uses two PAR ranges: blue (435-450nm) and red (640-665nm).
The key issue, in addition to selecting the appropriate wavelength, is a right proportion of red and blue light. Of course, other lengths spectrum of PAR are also used by plants, but the photosynthesis process occurs most intensively in the blue or red light.