Sunlight refers to the electromagnetic radiation of the sun that reaches the earth, particularly infrared, visible light, and ultraviolet light. The electromagnetic radiation consists of electromagnetic waves that can be characterized by frequency or wavelength of oscillations. However, not all of it reaches the earth. The earth's atmosphere filters the electromagnetic radiation allowing a part of it to pass through, especially visible light. Most of the sun's radiation does not reach the earth's surface. Some of it is absorbed in the atmosphere while others are scattered to space.
In terms of photosynthesis, the most essential part of the sun's radiation is the visible light. The visible light is that part of the electromagnetic radiation with wavelengths ranging between 380 nm and 750 nm. The length of the wavelength is inversely proportional to the amount of energy. It means that the shorter the wavelength, the higher the energy there is. Accordingly, red and blue of the visible light are the ones best absorbed by the plant pigment, chlorophyll, and also the ones most useful as energy for the light reactions of photosynthesis.