Is the Human Body a Continuous Spectrum
3. THE SPECTRUM OF RADIATION:
Almost all sources of electromagnetic radiation (except for some man-made sources such as radio transmitters or lasers) radiate photons of many different wavelengths at once. The most common examples are sunlight and the light from an incandescent light bulb, both of which consist of photons having many different wavelengths or colors -- all the colors of the rainbow (plus some invisible infrared and ultraviolet radiation). We can see that when we put white light through a spectrometer , a device that sends photons of different wavelengths in different directions, spreading the white light into a continuous spectrum , or rainbow. Isaac Newton made the pioneering experiments on the spectrum of light in 1667.
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| One kind of spectrometer is a prism, which bends blue light more than red light. White light is the combination of all visible colors. When white light passes through a prism, its different color components are separated into a rainbow. Source: University of Tennessee: Astronomy 162 . |
Your eyes (specifically, the cone cells in the retina), are capable of discerning among hundreds of colors. Nevertheless, your eyes are very imprecise devices for measuring spectra because they will recognize a beam of yellow (5800 A) and blue (4500 A) photons as green, and will not be able to distinguish such a beam from a beam of pure green (520 nm or 5200 A) photons. (A stands for Angstrom, or 10-10 meters. 1 A is about the diameter of a typical atom. 1 nanometer (nm) = 10 A.)
Blackbody Spectra
The most important kind of continuous spectrum is a blackbody spectrum or Planck Spectrum . It is the spectrum that a "perfect emitter" or a "perfect absorber" would radiate. We call it a blackbody because, for optical radiation, a black object is the closest approximation to a perfect absorber. A blackbody spectrum is an idealization -- very few objects radiate a perfect blackbody spectrum -- but many objects radiate a continuous spectrum that is a fairly good approximation to a blackbody spectrum. Examples are the Sun, the filament of a light bulb, and burning charcoal. The human body also radiates infrared radiation with a spectrum that is fairly close to a blackbody.
All blackbody spectra have the same typical shape, so that most of the radiation comes out as photons with wavelength clustered about some maximum. The only quality that distinguishes one blackbody spectrum from another is the temperature of the radiating surface. If the surface is hotter, the maximum shifts to shorter wavelength. This rule can be described exactly by a simple formula called Wien's Law . For example, the Sun, which has a temperature of about 5800 K, radiates a continuum spectrum that peaks at about 5000 A, right in the optical band. Your body, which has a temperature of 37 C, or 310 K (if you are healthy), radiates a continuum that peaks in the infrared band, at about 10-5 meters, or 10 micrometers. For an object at a temperature of 3 x 107 K (30 million degrees Kelvin), the peak is at about 1 A, in the X-ray band. You can find a nice Java demonstration of Blackbody Spectra here (from the University of Oregon).
Blackbody radiation has one other important property that is codified by the Stefan-Boltzmann Law .
Emission and Absorption Line Spectra
If stars and other objects in the universe radiated exactly as blackbodies, we would only be able to determine their temperatures and surface areas by remote sensing. Fortunately, their spectra are never perfect blackbodies. For example, the Sun's target="_top" spectrum , which is dominated by a continuum (rainbow) that resembles a blackbody, is punctuated by hundreds of dark lines * where photons of particular wavelengths are deficient or absent. All stars have such absorption line spectra (see below). (*called Fraunhofer lines, after Joseph Fraunhofer, who published a catalogue of about 600 such lines in 1812).
Other astronomical sources have emission line spectra , as illustrated above, where bright lines are present in the spectrum but little or no continuum.
Generally, objects of relatively high density, such as the gas at the surfaces of stars and solid objects, radiate continuum spectra. Emission and absorption lines are produced by gases at relatively low density. On Earth, we make emission line sources by sending electric currents through low-density gas in vacuum tubes. Often, street lights are made this way, using vaporized sodium or mercury in the vacuum tubes to emit the radiation.
By analyzing the emission and absorption lines in the spectra of stars (or other astronomical objects), we can learn much more about them than we could if their spectra consisted only of continuum radiation. You can think of a spectrum as a kind of "cosmic bar code", by which astronomers can read all sorts of detailed information identifying the star, just as the checkout machine at the supermarket can identify a product and its price from the bar code on the package. For example, we can determine what elements the stars are made of, the gas density of their atmospheres, the presence of magnetic fields, and how fast the stars are moving. To do this, we must understand how such spectral lines are formed. We do that by experiments in laboratories on Earth.
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Last modified January 18, 2002;
Copyright by Richard McCray
Source: https://jila.colorado.edu/~ajsh/courses/astr1120_03/text/chapter1/L1S3.html
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