© 2007, Ronald P. Walker
pinedaleairquality.com
"There are three stages in the life of a light beam: it is created, it travels through space, and it is destroyed...light is created and destroyed only via its interaction with matter, from glowing gases in the sun to rhodopsin in the eye." (Michael Sobin in "Light", p. 73, University of Chicago Press,1987). It is the interaction of light with electrons that is responsible for its creation and destruction.
Electrons in matter can be described as occupying energy levels. Electrons can be raised to higher energies by absorbing energy from a photon (destroying light) or move to lower energies by giving off a photon (creating light) whose energy is equal to the difference in energy between the two levels. Figure 1 shows two electron levels separated in energy by 2 electron volts (E = E2 - E1 = 2 eV).
Figure 1. Two electron energy levels in a solid: a) electron occupying the lower level and the upper level empty, b) photon is absorbed by the electron which moves to the upper level, c) photon is created by electron moving from upper to lower level with energy = E2-E1
An incident photon (orange light of 620 nanometers or 2 eV) gives up its energy (and vanishes) to the electron which moves to the upper level (Fig 1b). In Fig 1c, an electron in the upper level moves to the lower level by giving up an energy of 2 eV and emitting orange light. Thus, energy is conserved.
Electrons in atoms and in matter - from gases to solids - are held in place and require a boost in energy to remove them. In atoms the negatively charged electrons are held (bound) by the positively charged protons. We know that the electrons are bound to the atom with fixed energies called their binding energies.
In the case of light bulbs, light is emitted over the whole energy region of visible light by the heating of a tungsten filament inside the bulb. With electric discharge in gases light can also be produced in a spectrum of narrow (also called sharp) lines distributed in energy across the visible spectrum. Each line is characterized by the element atom which emits the light. Sodium, as an example, emits two lines fairly close together in the yellow region of the spectrum, cadmium emits a strong red and a strong green line, and mercury emits several strong lines at various colors.
Atomic emission spectroscopy can use flame excitation whereby atoms are excited by the heat of the flame to emit light. Energy is provided to a substance by the flame which is absorbed by the substance and boosts its electrons to a higher energy level. After a short period of excitation, the electrons drop back to their previous, more stable state known as the ground state and the absorbed energy is emitted. The wavelength and intensity of this emitted radiation can be measured, resulting in an electromagnetic spectrum. An emission spectrum of an element consists of several emission lines, each corresponding to a specific electron transition. Each of these lines, however is not perfectly monochromatic, meaning it does not have a single wavelength. It has a finite spread, which is called as the shape of the spectral line or spectral line broadening. Many atoms emit or absorb visible light and in order to obtain a fine line spectrum, the atoms must be in a gas phase which means that the substance has to be vaporized. Then, the spectrum can be studied in absorption or emission. (The emission technique is the mechanism by which the results of the flare spectroscopy presented in this web site was obtained. - RPW)
Optical instruments are used to measure the intensities and wavelengths of the visible region of the electromagnetic spectrum. A spectroscope is a "spectrum-observing" instrument and a spectrometer is for "spectrum measurement" and recording. A spectrometer actually measures the wavelength of light entering it.
The refraction and dispersion of light through a prism produces a spectrum of light from red to violet. Figure 3 shows a simple prism spectrometer for examining a light source
Figure 3. A simple prism spectrometer
with a focusing lens to produce parallel light (plane waves) which is dispersed by the prism. The spectrum of plane waves can be focused onto a photographic plate. The plate must be tilted to keep all the colors in focus because of the change in focal length of the lens with wavelength.
(In the case of the spectrometer used in the well flaring measurement project, the photographic plate is replaced by a sensitive array of CCD detectors. That detector array then produces an electrical signal which is processed and sent to a personal computer where appropriate software displays the signal as a waveform plotted in intesity versus wavelength format. The wavelength of the peaks appearing in this waveform can then be searched for their known chemical sources and the presence of those chemicals inferred in the flare. In truth, however, many peaks are due to complex molecules which emit energy in a complex manner, making their identification far more difficult. - RPW)
1. “Patterns in
Nature, Light and Optics, Optical Spectroscopy and Neon Lights,”
Authored by the
ACEPT W3
Group
Department of Physics
and Astronomy, Arizona State
University, Tempe, AZ 85287-1504
Copyright ©
1995-2000 Arizona Board of Regents. All rights reserved.
