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Sunlight and Life
| Article
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16600 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
11 / 1989 |
2,738 Words |
| Author
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Michael I. Sobel
Michael I. Sobel is professor of Physics at Brooklyn College
of the City University of New York and the author of Light, a
book for the nonspecialist. |
The interaction between light and the atom lies at the heart of many of the fundamental events in the natural and human world. Just as ordinary matter is constructed of packets of mass called atoms or molecules, light travels in the form of packets of energy called photons. An atom may exist in one of a set of possible energy states, and it can be driven from a lower to a higher energy state by absorbing a single photon, if the photon's energy just matches the energy difference between the two states of the atom. Driving an atom or molecule into a higher energy state may make possible certain chemical reactions that otherwise would not occur.
While humans cannot see a single photon or feel a single atom, photon energy is very familiar: For moving through the spectrum of rainbow colors, from red through orange, yellow, green, and blue to violet, simply corresponds to moving from low-energy photons to high-energy photons. Photons in the violet have almost twice the energy of photons in the red. Photons more energetic than those in the violet constitute what we call ultraviolet light; those less energetic than the red we call infrared light. Though the eye is not sensitive to these forms of energy, they are present in sunlight and play important roles in the story of life.
For many important organic molecules, the energy needed to move from the lowest energy state to a higher state corresponds to an ultraviolet photon. In the early earth, perhaps 3.8 billion years ago, just after the formation of the continents and the seas, ultraviolet photons may have been the agent that excited simple organic molecules, allowing them to react to form the complex molecules out of which the first one-celled creatures were formed.
Photosynthesizers
These earliest organisms would have relied on their chemical environment for food, taking in nonliving molecules to meet their needs for growth and reproduction. But living things could not begin their march toward the lush biosphere of today until a biological system developed that employed an external source of energy for these purposes. Although there are other energy sources present on earth (e.g., radioactivity and the ocean tides), the primary energy source is the light of the sun. The system that evolved was photosynthesis.
Sunlight is most intense in the visible part of the spectrum, so nature eventually designed molecules that absorb in the visible rather than the ultraviolet range. These
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