Radiation, blackbody direct

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [17]. The first is analogous to that in figure B1.3.3(b) where the action of the blackbody (- 2) is replaced by the action of a previously produced anti-Stokes wave, with frequency 03. This radiation actually experiences an attenuation since the value of Im x o3 ) is positive (leading to a negative gam coefficient). This is known as the stimulated Raman loss (SRL) spectroscopy [76]. Flowever the second source of anti-Stokes polarization relies on the presence of Stokes radiation [F7]. This anti-Stokes radiation will emerge from the sample in a direction given by the wavevector algebra = 2k - kg. Since the Stokes radiation is... 

A planar polished surface reflects heat radiation in a similar manner with which it reflects light. Rough surfaces reflect energy in a diffuse manner hence radiation is reflected in all directions. A blackbody absorbs all incoming radiation and therefore has no reflection. A perfect blackbody does not exist a near perfect blackbody surface such as soot reflects 5% of the radiation, making it the standard for an ideal radiator. 

But light is also a particle. Some properties of light cannot be explained by the wave-like nature of light, such as the photoelectric effect and blackbody radiation (see Section 9.4), so we also need to think of light comprising particles, i.e. photons. Each photon has a direction as it travels. A photon moves in a straight line, just like a tennis ball would in the absence of gravity, until it interacts in some way (either it reflects or is absorbed). 

Investigations on the emission properties of INSs started quite a long time ago, mainly in connection with the X-ray emission from PSRs. In the seventies it was a common wisdom that the radiation emitted by INSs comes directly from their solid crust and is very close to a blackbody. Lenzen and Trumper (1978) and Brinkmann (1980) were the first to address in detail the issue of the spectral distribution of INS surface emission. Their main result was that... 

The measurement of the spectral distribution of solar radiation outside the atmosphere and the subsequent association of this spectral distribution with the spectral distribution of radiation in a blackbody cavity has, I believe, biased the attempts to characterize the actual radiation in the atmosphere to an undue extent. Figure 1 indicates typical spectral distributions of radiation in the atmosphere as compared to that of solar radiation outside the atmosphere. Outside the atmosphere m 0 and if the flux is directly through m 1. If slanted at and angle from the zenith angle 90, then m is approximately 1/cos 60. 

When studying the limits of solar energy conversion, either by thermal or quantum processes, the sun has traditionally been treated as a blackbody (thermal equilibrium) radiator with surface temperature 5 800 K and distance 1.5 x 1011 m from Earth. A blackbody absorbs all incident radiation irrespective of its wavelength and direction of incidence and is represented classically by a hole in a cavity. 

There is a maximum amount of radiant energy emitted by a body at a given absolute temperature T at a wavelength X. This maximum amount of radiant emission is the spectral blackbody radiation intensity Ixb(T) the emitter of such radiation is named a blackbody. This spectral blackbody radiation intensity is independent of direction. For a blackbody at an absolute temperature T and emitting radiative energy into a vacuum, I. b(T) is calculated from the relation given by Planck, 1959 [1], in the form... 

A blackbody absorbs all incident radiation At all frequencies and from all different directions No phenomena of reflecting, transmitting or scattering It is emitting as much as it is absorbing. 

Thermodynamic considerations show that an ideal thermal radiator, or blackbody, will emit energy at a rate proportional to the fourth power of the absolute temperature of the body and directly proportional to its surface area. Thus... 

When Planck used his equation to calculate the spectrum of blackbody radiation, he came up with a result that agreed perfectly with experimental results. More importantly, he had discovered quantum mechanics, because this simple equation forms the basis of quantum theory. When applied to the physics of blackbodies, it implies that energy is not continuous but comes in tiny, irreducible packets, or quanta (a word coined by Planck himself), that are directly proportional to the frequency of an oscillator. 

A blackbody is defined as a perfect emitler and absorber of radiation. At a specified temperature and wavelength, no surface c.in emit more energy than a blackbody. A blackbody absorbs all incident radiation, regardless of wavelength and direction. Also, a blackbody emits radiation energy unifomily in all directions per um t area normal to direction of emission (Fig. 12-7). That is, a blackbody is a diffuse emitter. The tenti diffitse means independent of direction. The radiation energy emitted by a blackbody per unit time and per unit surface area was determined experimentally by Joseph Stefan in 1879 and expressed as... 

A blackbody is said to be a diffuse emitler since it emits radiation energy uniformly in all directions. 

If all surfaces emitted radiation uniformly in all directions, the emissive power would be sufficient to quantify radiation, and we would not need to deal with intensity. The radiation emitted by a blackbody pet unit nonnal area is the same in all directions, and thus there is no directional dependence. But this is not the case for real surfaces. Before we define intensity, wc need to quantify the size of an opening in space. 

This value of intensity is the same in ail directions since a blackbody i.s a diffuse emitter. Intensity represents the rale of radiation emission per unit area normal to the direction of emission per unit solid angle. Therefore, the rale of radiation energy emitted by A, in the direction of di through the solid angle is determined by multiplying /] by the area of Aj normal to 0i and the solid angle oij.,. That is,... 

The emissivity of a real surface is not a constant. Rather, it varies with the temperature of the surface as well as the wavelength and tlie direction of the emitted radiation. Therefore, different emissiviiies can be defined for a surface, depending on the effects considered. The most elemental emissivity of a surface at a given lemperature is the spectral directional emissivity, which is defined as the ratio of the intensity of radiation emitted by the suiface at a specified wavelength in a speeiOed direction to the intensit) of radiation emitted by a blackbody at the same temperature at the same wavelength. That is. 

A few comments about the validity of tlie diffuse approximation are in order. Although real surfaces do not emit radiation in a perfectly diffuse manner as a blackbody does, they often come close. The variation of emissivity with direction for both electrical conductors and nonconductors is given in Fig. 12 26. Here 0 is tlie angle measured from the normal of the surface, and thus 0 = 0 for radiation emitted in a direction normal to the surface. Note that Sg remains nearly constant for about 0 < d0° for conductors such as metals and for 6 < 70° for nonconductors such as plastics. Therefore, the directional emissivity of a sur face in the normal direction is representative of the hemispherical emissivity of the surface. In radiatioit analysis, it is common practice to assume the surfaces to be diffuse emitters with an emissivity equal to the value in the normal (6 = 0) direction. 

The direction of the net radiation heat transfer depends on the relative magnitudes of 7, (the radiosity) and (, (the emissive power of a blackbody at the teinpeiature of the surface). It is from the surface if > 7,- and to the surface if 7f > ft),-. A negative value for ft indicates that heat transfer is to the surface. All of this radiation energy gained must be removed from the other side of the surface through some mechanism if the surface temperature is to remain constant. 

The surface resistance to radiation for a blackbody is zero since e, = 1 and f -- E, The net rale of radiation heat transfer in this case is determined directly from Eq. 13-23. 

Two parallel disks of diameler D = 0.6 m separated by i. = 0.4 m are located directly on top of each other. Both disks are black and are maintaiiied at a temperature of 450 K. The back sides of the disks are insulated, and the environment that Ihe disks are in can be considered to be a blackbody at 300 K. Deierinine the nel rate of radiation heat transfer from the disks to the environment. Answer 781 W... 

For real surfaces emissivity is defined as the ratio of the radiation emitted by the surface to the radiation emitted by a blackbody at the same temperature. So, the emissivity specifies how well a real body radiates energy as compared with a blackbody. The directional spectral emissivity ex,e X, 9, , T) of a surface at temperature T is defined as the ratio of the intensity of the radiation emitted at the wavelength A and the direction of 9 and to the intensity of the radiation emitted by a blackbody at the same values of T and... 

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