The electromagnetic radiation are broadly classified into the following classes: gamma radiation, x-ray radiation, ultraviolet radiation, visible radiation, infrared radiation, microwave radiation, and radio waves. This classification goes in the increasing order of wavelength, a characteristic of the type of radiation.
Although in general, the classification scheme is accurate, in reality, there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz could be received and studied by astronomers, or maybe ducted along wires like electric power, although the latter is, in the strict sense, not electromagnetic radiation at all--the near field (or near field) and far field (or far field) and the transition zone are regions of the electromagnetic field around any object.
The distinction between X-rays and gamma rays is based on sources: gamma rays are the photons generated from nuclear decay or other nuclear and sub-nuclear/particle process, whereas X-rays are generated by electronic transition involving highly energetic inner atomic electrons. Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing radiation).
In general, nuclear transitions are much more energetic than electronic transitions, so gamma rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays even though their energy may exceed 6 mega electron volts (0.96 pJ), whereas there are many (77 known to be less than 10 keV (1.6 fJ)) low-energy transitions (e.g. the 7.6 eV (1.22 aJ) nuclear transition of thorium-229, where thorium is a natural radioactive chemical element with the symbol Th and atomic number 90), and, despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin.
Also, the region of the spectrum of the particular electromagnetic radiation is reference-frame dependent, which means it refers to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an “observational frame” tied to the state of motion of an observer. It is in the account of the lights’ Doppler shift, which was named after Austrian physicist Christian Doppler who proposed in 1842 in Prague that the change in frequency of a wave for an observer moving is relative to the source of the wave. Thus, EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum.
For example, consider the cosmic microwave background (CMB) radiation, the thermal radiation filling the observable universe almost uniformly. It was produced, when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons from Luyman series transitions, the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1 (where n is the principal quantum number referring to the energy level of the electron), putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly compared to the speed of light with respect to the cosmos. Red shift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum. However, for particles moving near the speed of light, this radiation will be blue-shifted in the rest frame. Blue shit, the opposite effect of redshift, is any decrease in wavelength or increase in frequency.
The highest-energy cosmic ray protons are moving such that, in their rest frame, this radiation is blue-shifted to high-energy gamma rays, which interact with the proton to produce bound quark-antiquark pairs, or pions (short for pi mession, denoted with π), any of the three subatomic particles π0, π+, and π−. This is the source of the Greisen-Zatsepin-Kuzmin limit (GZK limit), a theoretical upper limit on the energy of cosmic rays, or high energy charged particles from space, coming from “distant” sources.
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