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3 Photosynthesis

LIGHT

The Sun is the universal source of energy in the biosphere. During the nuclear fusion processes
occurring in the Sun, matter is changed into energy, which is emitted into space in the form of
electromagnetic radiation, having both wave and particle properties. The electromagnetic radiation
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has a spectrum or wavelength distribution from short wavelength (10 nm, g- and x-rays) to long
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wavelength (10 nm, long radio waves). About 99% of the Sun radiation is in the wavelength
region from 300 to 4000 nm and it is called the broadband or total solar radiation. Within this
broadband, different forms of energy exist, which can be associated with specific phenomena
such as harmful and potentially mutagen ultraviolet radiation (UV 100–400 nm), sight (visible
light 400–700 nm), and heat (infrared radiation 700–4000 nm). The particles producing the elec-
tromagnetic waves are called photons or quanta. The energy of a photon or quantum can be
expressed as hn, where h is the Planck’s constant (6.626 10 234 J sec) and n is the frequency of
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the photon. The frequency is in turn equal to cl 21 , where c is the speed of light (3 10 m sec 21 )
and l is the wavelength of the photon in nanometres (nm). According to this formula the shorter
the photon wavelength, the higher its energy; for example, the energy of one photon of 300 nm
light is 6.63 10 219 J, the energy of one photon of 400 nm light is 4.97 10 219 J, the energy of
one photon of 700 nm light is 2.84 10 219 J, and the energy of one photon of 4000 nm light is
0.49 10 219 J.
The energy of photons can also be expressed in terms of electron volts (eV). Absorption of a
photon can lead to excitation of an electron and hence of a molecule. This excited electron acquires
potential energy (capacity of producing chemical work) measured in eV. An electron volt is the
potential energy of 1 V gained by the excited electron, which is equal to 1.60 10 219 J. Thus
the energy of one photon of 300 nm light is equal to 4.14 eV, the energy of one photon of
400 nm light is equal to 3.11 eV, the energy of one photon of 700 nm light is equal to 1.77 eV,
and the energy of one photon of 4000 nm light is equal to 0.30 eV.
The average intensity of the total solar radiation reaching the upper atmosphere is about
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1.4 kW m (UV 8%, visible light 41%, and infrared radiation 51%). The amount of this energy
that reaches any one “spot” on the Earth’s surface will vary according to atmospheric and meteor-
ological (weather) conditions, the latitude and altitude of the spot, and local landscape features that
may block the Sun at different times of the day. In fact, as sunlight passes through the atmosphere,
some of it is absorbed, scattered, and reflected by air molecules, water vapor, clouds, dust, and
pollutants from power plants, forest fires, and volcanoes. Atmospheric conditions can reduce
solar radiation by 10% on clear, dry days, and by 100% during periods of thick clouds. At sea
level, in an ordinary clear day, the average intensity of solar radiation is less than 1.0 kW m 22 ,
(UV 3%, visible light 42%, infrared radiation 55%). Penetrating water, much of the incident
light is reflected from the water surface, more light being reflected from a ruffled surface than a
calm one and reflection increases as the Sun descends in the sky (Table 3.1). As light travels
through the water column, it undergoes a decrease in its intensity (attenuation) and a narrowing
of the radiation band is caused by the combined absorption and scattering of everything in the

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