ABSORPTION: LAMBERT–BEER LAW
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 (mol l-1), 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.
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.
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