## Each Photon of Light Has a Defined Amount of Energy

Quantum mechanics established that light, a form of electromagnetic radiation, has properties of both waves and particles. When light interacts with matter, it behaves as discrete packets of energy (quanta) called photons. The energy of a photon, e, is proportional to the frequency of the light wave: e = hy, where h is Planck's constant (1.58 X 10"34 cal-s, or 6.63 X 10"34 J-s) and y is the frequency of the light wave. It is customary in biology to refer to the wavelength of the light wave, X, rather than to its frequency y. The two are related by the simple equation y = c — X, where c is the velocity of light (3 X 1010 cm/s in a vacuum). Note that photons of shorter wavelength have higher energies.

Also, the energy in 1 mol of photons can be denoted by E = Ne, where N is Avogadro's number (6.02 X 1023 molecules or photons/mol). Thus

The energy of light is considerable, as we can calculate for light with a wavelength of 550 nm (550 X 10"7 cm), typical of sunlight:

(6.02 X 1023 photons/mol) (1.58 X 10"34cal■ s) (3 X 1010 cm/s)

550 X 10~7cm

51,881 cal/mol or about 52 kcal/mol. This is enough energy to synthesize several moles of ATP from ADP and Pi if all the energy were used for this purpose. 