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Paul Shashkov
Paul Shashkov

Blackbody Extra Quality

A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body also emits black-body radiation. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."[1]


An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation.[8]

There is interest in blackbody-like materials for camouflage and radar-absorbent materials for radar invisibility.[30][31] They also have application as solar energy collectors, and infrared thermal detectors. As a perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater, particularly in space or in a vacuum where convective heating is unavailable.[32] They are also useful in telescopes and cameras as anti-reflection surfaces to reduce stray light, and to gather information about objects in high-contrast areas (for example, observation of planets in orbit around their stars), where blackbody-like materials absorb light that comes from the wrong sources.

A blackbody refers to an opaque object thatemits thermal radiation. A perfectblackbody is one that absorbs all incoming light and does notreflect any. At room temperature, such an object wouldappear to be perfectly black (hence the termblackbody). However, if heated to a hightemperature, a blackbody will begin to glow withthermal radiation.

Fortunately, it is possible to construct a nearly-perfect blackbody.Construct a box made of a thermally conductive material, such asmetal. The box should be completely closed on all sides, so that theinside forms a cavity that does not receive light from thesurroundings. Then, make a small hole somewhere on the box.The light coming out of this hole will almost perfectly resemble thelight from an ideal blackbody, for the temperature of the air insidethe box.

At the beginning of the 20th century, scientists Lord Rayleigh,and Max Planck (among others) studied the blackbodyradiation using such a device. After much work, Planck was able toempirically describe the intensity of light emitted by a blackbody as afunction of wavelength. Furthermore, he was able to describe how thisspectrum would change as the temperature changed. Planck's work onblackbody radiation is one of the areas of physics that led to thefoundation of the wonderful science of Quantum Mechanics, but that isunfortunately beyond the scope of this article.

What Planck and the others found was that as the temperature of ablackbody increases, the total amount of light emitted persecond increases, and the wavelength of the spectrum's peak shifts tobluer colors (see Figure 1).

where T is the temperature in Kelvin. Wien's law (also known asWien's displacement law) states that thewavelength of maximum emission from a blackbody is inverselyproportional to its temperature. This makes sense;shorter-wavelength (higher-frequency) light corresponds tohigher-energy photons, which you would expect from ahigher-temperature object.

where A is the surface area, alpha is a constant of proportionality,and T is the temperature in Kelvin. That is, if we double thetemperature (e.g. 1000 K to 2000 K) then the total energy radiatedfrom a blackbody increase by a factor of 24 or 16.

The CMB has the spectrum of a blackbody. A blackbody spectrum is produced by an isothermal,opaque and non-reflecting object. Usually a cavity with a small holeis used in the laboratory to make an opaque and non-reflective object.Radiation that enters the cavity through the hole will have to bounceoff many walls before it returns to the outside, so even if the wallsare only somewhat dark, the hole will appear to be completely black.The diagram at right shows such a cavity, with the blue incoming raybeing absorbed completely while the red rays show the outgoing thermalradiation. A simple gedanken experiment shows that thespectrum emitted by a blackbody can only depend on its temperature T.The proof first assumes that two blackbodies have different spectra andthen shows that this leads to a contradiction. Let two blackbodies A& B, both at temperature T, radiate different spectra. Then use afilter and aperture stops to allow them to transfer heat only byradiation in a given passband. Then the radiation of A is entirelyabsorbed by B, and the radiation of B is entirely absorbed by A. Thusif their spectra are different, there would be a net transfer of heatbetween A & B, but their temperatures are the same. Since heattransfer between objects of the same temperature does not occur, thespectra must be identical. The choice of filter passband wasarbitrary, so the spectra must be identical at all frequencies. Thisuniversal blackbody spectrum was clearly a very important topic inphysics at the end of the 19th century, and Planck was studyingblackbody radiation when he introduced the idea of quanta, anddefined the quantum of action h which we now know as Planck'sconstant. Because of the universality of the blackbody spectrum, wecan convert any spectral measurement into a brightnesstemperature at the measured wavelength. The unique character of ablackbody spectrum is that the brightness temperature of a blackbody is thesame at all wavelengths.When talking about the CMB scientists always use the Kelvin scale oftemperature, which is just like the Celsius scale except the zero pointis absolute zero instead of the freezing point of water at Tice = 273.15 K.The graph above shows the measured brightness temperature TB of the CMB at many different wavelengths. Clearly TB = 2.725 K is consistent with all the datawithin the statistical scatter expected for the stated errors.

In order to make a blackbody spectrum, an object as to be opaque,non-reflective and isothermal. Thus a star, which is opaque, does notproduce a blackbody spectrum because we can see both cooler outerlayers and hotter deeper layers. But even though the temperature ofthe Universe changes as it evolves, with TCMB =To (1+z), the Universe looks isothermal because theredshifting of radiation makes the warmer but redshifted distantUniverse appear to have exactly the same temperature as the Universetoday.

The FIRAS instrument on COBE had a large conical horn for collectingthe cosmic microwave background. There was only a small hole in theend of the horn to let the radiation into the instrument. But FIRASalso carried a microwave absorber, the external calibrator or XCAL,that could be inserted into the horn like a trumpet mute, and heatersthat could make the whole horn+plus absorber cavity isothermal. Whenthe XCAL was in the horn FIRAS observed a very good blackbody cavity,but when the XCAL was out FIRAS observed the CMB. No signifcantdifference could be seen. The CMB is very close to a blackbody withtemperature 2.725 K.The FIRAS results are shown below in units of intensity (power perunit area per unit frequency per unit solid angle) vs.frequency and/or wavelength.Eric Adelbergerwould like me to point out that the fundamental FIRASmeasurement is the residual plot at the bottom. This is what FIRASactually measured: the difference between the CMB and the best fittingblackbody.The plot at top shows this residual added to the theoretical blackbodyspectrum at the best fitting XCAL temperature, based on the functionderived by Planck in 1900. The three curves in the bottom correspondto three fairly likely non-blackbody spectra: the grey curve shows abody with a reflectivity of 100 parts per million instead of zero, andthe red and blue curves show the effect of hot electronsadding an excess 60 parts per million of energy to the CMB either before(blue) or after (red) 1000 years after the Big Bang. These curves showthe maximum distortions allowed by the FIRAS data.

Penzias & Wilson were studying the radiation collected by a largehorn antenna in New Jersey when they found an excess radiation at 7.35 cmwavelength that wasequivalent to a 3.5 +/- 1 K blackbody. Their horn had lowsidelobes, and Penzias & Wilson switched against a low temperaturereference load. They did not know what this excess meant, but spoke toBernie Burke of MIT, who knew that Dicke was now leading a groupplanning to measure the CMB. Ironically Dicke had forgotten about hisold upper limit, but he knew how to do the measurement. But before hisgroup could finish building their instrument, Dicke got a call fromPenzias & Wilson. After hearing about their data, Dicke said:"Boys, we've been scooped." Papers by Penzias & Wilson and by Dicke, Peebles, Roll & Wilkinson describing these results appearedin the 1965 Astrophysical Journal. Wilkinson went on tomeasure the CMB at a large number of wavelengths and always found thesame brightness temperature over a wide range of wavelengths.

blackbody - an object which absorbs all radiation falling upon it, anddoes not reflect any. All radiation emitted by a blackbody is due to itstemperature. A star is a near perfect blackbody.

I. The amount of energy radiated by an object is related to its temperature. Thehotter the object, the more energy it releases. This idea is represented by theStefan-Boltzmann lawwhere Energy Flux (F) is the amount of energy emitted every second, σ (Greek symbol sigma) is the Stefan-Boltzmann constant (equal to 5.67 x 10-8 watts meter-2 K-4), and T is temperature in degrees kelvin (K). This law is best applied to a blackbody.The law says, for example, if you double an object's temperature, the amount of energy itreleases increases by a factor of 16. 041b061a72


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