|FIGURE 5.2 Light interaction with matter: the scattering process.
ABSORPTION: LAMBERT–BEER LAW
In the case of solar radiation, scattering is due to its interaction with gas molecules and suspended
particles found in the atmosphere. Scattering reduces the amount of incoming radiation
reaching the Earth’s surface because significant proportion of solar radiation is redirected back
to space. The amount of scattering that takes place is dependent on two factors: wavelength of
the incoming radiation and size of the scattering particle or gas molecule. For small particles
compared to the visible radiation, Rayleigh’s scattering theory holds. It states that the intensity
of scattered waves roughly in the same direction of the incoming radiation is inversely proportional
to the fourth power of the wavelength. In the Earth’s atmosphere, the presence of a large number of
small particles compared to the visible radiation (with a size of about 0.5 µm) results such that the
shorter wavelengths of the visible range are more intensely diffused. This factor causes our sky to
look blue because this color corresponds to those wavelengths. When the scattering particles are
very much larger than the wavelength, then the intensity of scattered waves roughly in the same
direction of the incoming radiation become independent of wavelength and for this reason, the
clouds, made of large raindrops, are white. If scattering does not occur in our atmosphere the
daylight sky would be black.
Some molecules have the ability to absorb incoming light. Absorption is defined as a process in
which light is retained by a molecule. In this way, the free energy of the photon absorbed by the
molecule can be used to carry out work, emitted as fluorescence or dissipated as heat.
The Lambert–Beer law is the basis for measuring the amount of radiation absorbed by a
molecule, a subcellular compartment, such as a chloroplast or a photoreceptive apparatus and a
cell, such as a unicellular alga (Figure 5.3). A plot of the amount of radiation absorbed (absorbance, Aλ
) as a function of wavelengths is called a spectrum. The Lambert–Beer law states that the
variation of the intensity of the incident beam as it passes through a sample is proportional to
the concentration of that sample and its thickness (path length). We have adopted this law to
measure the absorption spectra in all algal photosynthetic compartments.
The Lambert–Beer law states the logarithmic relationship between absorbance and the ratio
between the incident (II
) and transmitted light (IT
). In turn, absorbance is linearly related to the pigment concentration C
), the path length l
(cm) and the molar extinction coefficient ελ
, which is substance-specific and a function of the wavelength.
Table 5.1 shows the comparison between transmitted light and absorbance values.
|FIGURE 5.3 Light absorption by a unicellular alga: II, light incident on the cell and IT, light transmitted by the cell
Electromagnetic waves can superimpose. Scattered waves, which usually have the same frequency,
are particularly susceptible to the phenomenon of interference, in which waves can add constructively
or destructively. When two waves, vibrating in the same plane, meet and the crests of one
wave coincide, with the crests of the other wave, that is, they are in phase, then constructive interference
occurs. Therefore, the amplitude of the wave has been increased and this results in the light
appearing brighter. If the two waves are out of phase, that is, if the crests of one wave encounter the
troughs of the other, then destructive interference occurs. The two waves cancel out each other,
resulting in a dark area (Figure 5.4). The interference of scattered waves gives rise to reflection,
refraction, diffusion, and diffraction phenomena.
|FIGURE 5.4 Interference of light passing through two narrow slits, each acting as a source of waves. The
superimposition of waves produces a pattern of alternating bright and dark bands. When crest meets crest or
trough meets trough, constructive interference occurs, which makes bright bands; when crest meets trough
destructive interference occurs, which makes dark bands. The dots indicate the points of constructive
interference. The light intensity distribution shows a maximum that corresponds to the highest number of dots.