2 Solar Spectra
19
spectrum reaching the earth surface at an Air mass ratio of 1.5 (AM1.5) after having
interactions with the atmosphere.
The atmosphere interacts both with incoming sunlight as well as outgoing light
from earth. Two main processes take place: light scattering and light absorption.
Whereas light scattering redistributes any light energy in the atmosphere without
changing the wavelength characteristics, light absorption converts the light energy
to internal energy of the absorbing molecules and eventually transfers it to the surrounding atmosphere as heat. Now, the solar spectrum changes significantly in terms
of its spectral nature, as the sunlight passes through the atmosphere. This is due to
the fact that solar radiation is absorbed in the atmosphere by different interactions
with atmospheric constituents as shown by the black cure in the Fig. 2.1. The individual wavelengths of the spectrum have different extents of interaction with the
atmospheric constituents. Some of the effects and interactions of incoming sunlight
with the atmosphere are described hereunder:
(a) Reflection of light
Solar radiation is reflected in the atmosphere and this reduces the radiation reaching the Earth. Major reflection of sunlight takes place from the clouds by particles
of liquid state or frozen state of water. The reflection in such a situation can vary
from 40 to 90% depending on density of clouds and composition of clouds. The
other constituents of atmosphere also contribute to reflection, but their percentage of
reflection is small.
(b) Absorption of light
When the solar radiation interacts with the gases and particles in the atmosphere,
some of the latter have the ability to absorb incoming solar radiation. Absorption in
the atmosphere can be defined as a process in which solar radiation is captured by the
atmospheric constituents. This captured energy heats up these gases and particles.
The captured energy is then re-radiated in the form of heat or long-wavelength
radiation. A large variety of molecules in the atmosphere, e.g. water vapour, O 2 ,
O 3 , CO 2 molecules, etc. exhibit absorption in the region of the solar spectrum: Thus,
partial absorption of sunlight alters the nature of sunlight reaching the Earth’s surface.
Due to this process, some of the wavelengths are practically absent in the incident
sunlight, especially in the infrared region (above 800 nm wavelength) of the solar
spectrum as noted for the black curve in Fig. 2.1.
(c) Rayleigh scattering
When sunlight falls on particles that are smaller than its wavelength, then Raleigh
scattering takes place. Rayleigh scattering is strongly dependent on the wavelength
λ; it is proportional to 1/λ
4
. Therefore, the amount of Rayleigh scattering increases
as the wavelength decreases. This is the reason why the shorter wavelengths of
sunlight i.e. blue light are strongly scattered, which is why the sky is blue and the
sea is blue. The Rayleigh scattering phenomenon also explains the fact that the solar
spectrum has a higher content of red and infrared in the morning and in the evening,
19
spectrum reaching the earth surface at an Air mass ratio of 1.5 (AM1.5) after having
interactions with the atmosphere.
The atmosphere interacts both with incoming sunlight as well as outgoing light
from earth. Two main processes take place: light scattering and light absorption.
Whereas light scattering redistributes any light energy in the atmosphere without
changing the wavelength characteristics, light absorption converts the light energy
to internal energy of the absorbing molecules and eventually transfers it to the surrounding atmosphere as heat. Now, the solar spectrum changes significantly in terms
of its spectral nature, as the sunlight passes through the atmosphere. This is due to
the fact that solar radiation is absorbed in the atmosphere by different interactions
with atmospheric constituents as shown by the black cure in the Fig. 2.1. The individual wavelengths of the spectrum have different extents of interaction with the
atmospheric constituents. Some of the effects and interactions of incoming sunlight
with the atmosphere are described hereunder:
(a) Reflection of light
Solar radiation is reflected in the atmosphere and this reduces the radiation reaching the Earth. Major reflection of sunlight takes place from the clouds by particles
of liquid state or frozen state of water. The reflection in such a situation can vary
from 40 to 90% depending on density of clouds and composition of clouds. The
other constituents of atmosphere also contribute to reflection, but their percentage of
reflection is small.
(b) Absorption of light
When the solar radiation interacts with the gases and particles in the atmosphere,
some of the latter have the ability to absorb incoming solar radiation. Absorption in
the atmosphere can be defined as a process in which solar radiation is captured by the
atmospheric constituents. This captured energy heats up these gases and particles.
The captured energy is then re-radiated in the form of heat or long-wavelength
radiation. A large variety of molecules in the atmosphere, e.g. water vapour, O 2 ,
O 3 , CO 2 molecules, etc. exhibit absorption in the region of the solar spectrum: Thus,
partial absorption of sunlight alters the nature of sunlight reaching the Earth’s surface.
Due to this process, some of the wavelengths are practically absent in the incident
sunlight, especially in the infrared region (above 800 nm wavelength) of the solar
spectrum as noted for the black curve in Fig. 2.1.
(c) Rayleigh scattering
When sunlight falls on particles that are smaller than its wavelength, then Raleigh
scattering takes place. Rayleigh scattering is strongly dependent on the wavelength
λ; it is proportional to 1/λ
4
. Therefore, the amount of Rayleigh scattering increases
as the wavelength decreases. This is the reason why the shorter wavelengths of
sunlight i.e. blue light are strongly scattered, which is why the sky is blue and the
sea is blue. The Rayleigh scattering phenomenon also explains the fact that the solar
spectrum has a higher content of red and infrared in the morning and in the evening,