The energy produced by the sun reaches the earth as electromagnetic radiation. Light and other forms of electromagnetic radiation are considered to have both a wave nature and a particle nature. Particles or packets of light (its particle nature) are known as photons - the smallest divisible units of light. The brightness of light depends on the number of photons absorbed per unit time. Each photon carries a fixed amount of energy which determines the amount that the photon vibrates. The distance moved by a photon during one of it vibrations is referred to as its wavelength and is measured in nanometres.
Electromagnetic radiation spans a broad range of wavelengths. At the one end of the spectrum of electromagnetic radiation there are gamma rays which have a wavelength of 10-5 nm and at the other end, radio waves which have a wavelength of 1012nm. A very small part of this spectrum can be seen by the human eye i.e. between the wavelengths 380 and 750 nm. This part of the electromagnetic spectrum is called visible light. Almost all life depends ultimately on this part of the spectrum for its energy. Humans perceive the different wavelengths of visible light as different colours.
Within the spectrum the longer the wavelength of the radiation, the slower the vibration of the photons and the less energy each photon contains. Thus photons of ultraviolet light, at the blue end of the visible spectrum, have shorter wavelengths and contain more energy than red light and infrared radiation.
Sunlight contains 4% ultraviolet radiation, 52% infrared radiation and 44% visible light.
Why is only visible light used by plants ?
Chlorophyll does not absorb all the wavelengths of visible light equally. Chlorophyll a, the most important light-absorbing pigment in plants, does not absorb light in the green part of the spectrum. Light in this range of wavelengths is reflected. This is the reason why chlorophyll is green and also why plants (which contain a lot of chlorophyll) are also green. Note in the graph above that the absorption of light by chlorophyll a is at a maximum at two points on the graph 430 and 662 nm. The rate of photosynthesis at the different wavelengths of visible light also show two peaks which roughly correspond to the absorption peaks of chlorophyll a. Plants do not depend only on chlorophyll a in their light harvesting machinery but also have other pigments (accessory pigments) which absorb light of different wavelengths.