When a molecule interacts with light and energy is absorbed, the molecule is said be excited and a transition occurs which can take the molecule from an initial state to a higher energy state. Within the one-electron approximation, this is described by the promotion of an electron from a filled orbital to an unfilled orbital in the case of diamagnetic materials.
The difference in energy between those levels, the excited state and the ground state , gives the energy of the photons that can be absorbed. Several parameters can be used to characterize this transition, including the energy of the incident radiation required for the efficient absorption of the light and the inherent ability of the molecules to absorb radiation of the appropriate energy by the Planck relation:.
The energy is reported in several units; the following is helpful for translating between some common units one comes across in the literature:. Our perception of color is determined by what wavelengths of radiation reach our eye and the sensitivity of the receptors in our eye to various colors The eye has rods and cones containing chromophores which convert light into electrical impulse that the brain uses to perceive images.
This the opposite of what you see in light emitting diodes in which electricity causes emission of light. The rods function under low intensity conditions and provide images in shades of black, grey, and white This is referred to as scotopic vision. The cones process images of high intensity in color which is referred to as photopic vision.
Cones come in three varieties which correspond roughly to blue, green, and red sensitivities; if all three cones are simultaneously excited, then the image will appear white. Thus, we see that rotational energy is distributed over many rotational levels, but that vibrational and electronic levels are usually the ground lowest energy levels.
Atoms do not have molecule-like vibrations and rotations because they have only one nucleus, so their spectra consist only of electronic transitions.
A sharp transition from one discrete level to another, which appears as a line in a spectrum, occurs in a narrow range of energies about the transition energy difference. The resulting radiation occurs in a narrow band of wavelengths about the line's central wavelength. The width of this line measured at half the line's maximum height is called the linewidth. High in the atmosphere, Doppler broadening is dominant because the pressure is low, but lower in the atmosphere, pressure broadening becomes dominant even though Doppler broadening also increases.
An energy level diagram for a molecule. Not all transitions are allowed between all levels because of the conservation of angular momentum. Credit: UC Davis chemwiki. Calculated solar and terrestrial irradiances in the atmosphere. Gases considered for absorption are water vapor, carbon dioxide, oxygen, ozone, and nitrous oxide. Scattering is due to all gases but mainly nitrogen and oxygen. The top panel shows the downgoing solar radiation at the Earth's surface black line with red filling and the upgoing terrestrial radiation at the top of the atmosphere black line with blue filling.
Also shown in the top panel are the Planck spectral irradiances for representative solar and terrestrial temperatures. Note that all curves in the top panel have been scaled such that the peak values are roughly the same. The middle panel shows the total extinction due to absorption and scattering by gases in the atmosphere.
On the other hand, if certain wavelengths of colours are reflected from the material, an observer will see them and see the material in those colours. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green - leaves therefore appear green. Discover more about light absorption in our Learning Centre.
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