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The Nature of Light Energy

The energy for photosynthesis is derived from light. Although sunlight appears white it is actually a mixture of different colors. The electromagnetic spectrum consists of radiations of different wavelengths. It includes gamma rays, X-rays, ultraviolet rays, the visible spectrum, infrared rays and radio waves. Each of these types of waves has a characteristics range of wavelengths. The visible spectrum is constituted by the bands of color visible to the human eye. It ranges from 3,800-7,800A, and constitutes just a small region of the spectrum of electromagnetic radiations.

electormagnetic spectrum

According to the wave theory of light, each color of the spectrum consists of a different wavelength than violet light at the other end. The longer the wavelength the less the energy conveyed. Conversely, shorter wavelengths convey more energy. Thus red light with a longer wavelength than violet light conveys less energy than violet light.

The absorption spectrum. The amount of absorption of light at different wavelengths constitutes the absorption spectrum. If light of different wavelengths is passed through a chlorophyll extract it is possible to measure the absorption at each wavelength. An absorption spectrum of chlorophyll a, the main light absorbing pigment, shows that light is mainly absorbed in the blue and red regions.
Filaments of the green alga Spirogyra were placed on a slide. They are illuminated through a prism with the spectrum of light. A drop of water containing hundreds of aerobic bacteria was placed on the algal filaments. These bacteria depend upon oxygen for respiration. It was found that after a few minutes the bacteria clustered around the filaments and were concentrated in the blue and red regions. It was in these regions of light that maximum oxygen was available, indicating maximum photosynthetic activity.

leaf reflaction

Excitation of electrons. As mentioned previously, light comes in the form of packets or photons. When an atom absorbs a photon it is said to be in an ‘activated’ or ‘excited’ state. Such excitation creates an unstable condition within the molecule. All atoms contain electrons which travel around the nucleus in one or more orbits.

Light energy absorbed by chlorophyll can be released as fluorescence, phosphorescence or heat (waste), or can be released for useful work. When a photon of light is absorbed by an atom in the low energy state, the electron is pushed into a high energy orbit. This orbit is further away from the nucleus. The electron therefore becomes unstable or excited, and is said to be in the singlet state. The electron now behaves in one of the following ways: (1) It can return to the low energy ground state, releasing the absorbed energy. This emission of radiant energy in visible from is called fluorescence. (2) The excited high-energy electron can drop into a slightly lower energy state called the triplet state. (3) The electron can further drop the triplet state back to the ground state. Here also energy emission in visible from take place, and is called phosphorescence. (4) The electron can also drop from the excited state to the ground state with the release of heat.

When a photon of light strikes a chlorophyll molecule it can transfer its energy to the outer electron. This electron now becomes excited and is raised to a higher energy level.

changes in chlorophyll

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