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γ = Gamma Ray s HX = Hard X-ray s SX = Soft X-Rays EUV = Extreme Ultraviolet NUV = Near ultraviolet Visible Light NIR = Near Infrared MIR = Moderate infrared FIR = Far infrared Radio Waves EHF = Extremely High Frequency (Microwaves) SHF = Super High Frequency (Microwaves) UHF = Ultra High Frequency VHF = Very High Frequency HF = High Frequency MF = Medium Frequency LF = Low Frequency VLF = Very Low Frequency VF = Voice Frequency ELF = Extremely Low Frequency ]] The electromagnetic (EM) spectrum is the range of all possible Electromagnetic Radiation . The "electromagnetic spectrum" (usually just ''spectrum'') of an object is the frequency range of electromagnetic radiation with wavelengths from thousands of Kilometre s down to fractions of the size of an Atom . It is commonly said that EM waves beyond these limits are uncommon, although this is not actually true. The short wavelength limit is likely to be the Planck Length , and the long wavelength limit is the size of the Universe itself (see Physical Cosmology ), though in principle the spectrum is Infinite . Electromagnetic energy at a particular Wavelength λ (in vacuum) has an associated Frequency ''f'' and Photon Energy ''E''. Thus, the electromagnetic spectrum may be expressed equally well in terms of any of these three quantities. They are related according to the equations: wave speed or : and : or : where:
So, high-frequency electromagnetic waves have a short wavelength and high energy; low-frequency waves have a long wavelength and low energy. When light waves (and other electromagnetic waves) enter a medium, their Wavelength is reduced. Wavelengths of electromagnetic radiation, no matter what medium they are travelling through, are usually quoted in terms of the ''vacuum'' wavelength, although this is not always explicitly stated. SPECTRA OF OBJECTS Nearly all objects in the Universe emit, reflect or transmit some light. (One hypothetical exception may be Dark Matter , which, along with Dark Energy make up 96% of the universe's total mass.) The distribution of this light along the electromagnetic spectrum (called the ''spectrum'' of the object) is determined by the object's composition. Several types of spectra can be distinguished depending upon the nature of the radiation coming from an object:
Electromagnetic Spectroscopy is the branch of physics that deals with the characterization of matter by its spectra. A Halogen incandescent lamp, has a light spectra as shown in the accompanying figure. The figure shows just the infrared end of the whole spectra, which is limited by the resolution of the spectrum analyzer from 600nm to 1500nm, in the optical window, typical to fiber optic communication systems. WAVE CHARACTERISTICS EXTERNAL LINKS
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