From quantum mechanical theory, we observe that

= c/,

where is the wavelength of light, is its frequency, and c is the speed of light in a vacuum.The energy per photon is given by Planck's law,

E = h,

where E is the energy and h is Planck's constant (6.626 x 10^-34 J s). Figure 17.6 shows the energy per mole of photons (one einstein) versus wavelength. The range of sensitivity of photosynthetic pigments is shown, as is the vibrational energy of molecules and a few common covalent bonds. Note also that the range of sensitivity of photosynthetic pigments corresponds to the span of wavelengths received at the surface of the Earth (Figure 17.8).

Photosynthesis depends primarily on light in the visible and near-infrared regions of the spectrum. Light of shorter wavelengths can break covalent bonds and is thus too strong to be practical for use in photosynthesis. Also, ultraviolet light can penetrate only a very short distance into water and is thus unavailable to photosynthetic organisms living in the sea. Wavelengths longer than visible light (i.e., infrared radiation), on the other hand, contain too little energy to be of much use for photosynthesis.