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Quite often in science, the most intriguing phenomena are the results of the simplest processes. And the same holds true for the multitude of radiation we witness around us. It all starts with a simple phenomenon - oscillation of charges! An oscillating electric charge leads to an oscillating electric field, which by virtue of Maxwell’s equations, produces an oscillating magnetic field perpendicular to the electric field. These electromagnetic fields oscillate at the same frequency as the charge that has produced it. The oscillating fields can propagate through free space (vacuum) or any medium and this is what we term as Electromagnetic Waves (Fig. 1).
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| Fig. 1: Propagation of electromagnetic wave |
The number of cycles the electric (or magnetic) field completes per second is called the frequency of the electromagnetic wave. The distance travelled by the wave in one cycle is called the wavelength of the wave. Although these waves travel at the speed of light (in vacuum), they do so at different frequencies, wavelengths and energies. And depending on the frequency (or wavelength), the electromagnetic waves can be distributed in what constitutes the electromagnetic spectrum. The spectrum comprises the span of all electromagnetic radiations, ranging from longest waves (radio waves) with frequencies in the range of a few Hz to shortest waves (gamma rays) with frequencies of the order of 1025 Hz. The entire division of the EM waves within the spectrum is as illustrated in Fig. 2.
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| Fig. 2: The electromagnetic spectrum |
Emission spectrum
Emission spectra are produced by thin gases in which the atoms do not experience many collisions (because of the low density). The emission lines correspond to photons of discrete energies that are emitted when excited atomic states in the gas make transitions back to lower-lying levels.
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| Fig. 3. Origin of continuous, emission and absorption spectra |
Continuum spectrum
A continuum spectrum results when the gas pressures are higher, so that lines are broadened by collisions between the atoms until they are smeared into a continuum. We may view a continuum spectrum as an emission spectrum in which the lines overlap with each other and can no longer be distinguished as individual emission lines.
Absorption Spectrum
An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum.
The emission and absorption spectra for the same gas appear complementary to each other.
In addition to spectra associated with atoms and ions, molecules can interact with electromagnetic radiation and give rise to characteristic spectra. Because of basic atomic and molecular structure, the spectra associated with molecules typically involve infrared wavelengths.
The world around us is jammed with electromagnetic radiation. Almost all interactions that we experience in our daily lives are the manifestation of electromagnetic interactions, i.e. the interaction of light (EM waves) with matter. As a result, the study of electromagnetic waves and interactions has been the pivot for an immense number of advancements in science and technology. Thus, it would not be an exaggeration to state that electromagnetic waves have illuminated our past, are enlightening the present, and will continue to brighten up our future.
REFERENCES
- https://www.pas.rochester.edu/~blackman/ast104/absorption.html
- https://www.britannica.com/science/electromagnetic-spectrum
- www.rfcafe.com (For Fig. 1)
- www.thecolouragency.com (For Fig. 2)
- https://www.pas.rochester.edu/~blackman/ast104/absorption.html (For Fig. 3)
