Blackbody Radiation. A blackbody is a surface that completely absorbs all incident radiation
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1 Blackbody Radiation
2 Blackbody Radiation A blackbody is a surface that completely absorbs all incident radiation
3 Blackbody Radiation A blackbody is a surface that completely absorbs all incident radiation emits radiation at the maximum possible monochromatic intensity in all directions and at all wavelengths.
4 Blackbody Radiation A blackbody is a surface that completely absorbs all incident radiation emits radiation at the maximum possible monochromatic intensity in all directions and at all wavelengths. The theory of the energy distribution of blackbody radiation was developed by Planck and first appeared in 1901.
5 Blackbody Radiation A blackbody is a surface that completely absorbs all incident radiation emits radiation at the maximum possible monochromatic intensity in all directions and at all wavelengths. The theory of the energy distribution of blackbody radiation was developed by Planck and first appeared in Planck postulated that energy can be absorbed or emitted only in discrete units or photons with energy E = hν = ω The constant of proportionality is h = J s.
6 Planck showed that the intensity of radiation emitted by a black body is given by B λ = where c 1 and c 2 are constants c 1 λ 5 exp(c 2 /λt ) 1 c 1 = 2πhc 2 = W m 2 and c 2 = hc k = m K. The function B λ is called the Planck function. 2
7 Planck showed that the intensity of radiation emitted by a black body is given by B λ = where c 1 and c 2 are constants c 1 λ 5 exp(c 2 /λt ) 1 c 1 = 2πhc 2 = W m 2 and c 2 = hc k = m K. The function B λ is called the Planck function. For a derivation of the Planck function, see for example the text of Fleagle and Businger, Atmospheric Physics. 2
8 Planck showed that the intensity of radiation emitted by a black body is given by B λ = where c 1 and c 2 are constants c 1 λ 5 exp(c 2 /λt ) 1 c 1 = 2πhc 2 = W m 2 and c 2 = hc k = m K. The function B λ is called the Planck function. For a derivation of the Planck function, see for example the text of Fleagle and Businger, Atmospheric Physics. Blackbody radiation is isotropic. 2
9 Planck showed that the intensity of radiation emitted by a black body is given by B λ = where c 1 and c 2 are constants c 1 λ 5 exp(c 2 /λt ) 1 c 1 = 2πhc 2 = W m 2 and c 2 = hc k = m K. The function B λ is called the Planck function. For a derivation of the Planck function, see for example the text of Fleagle and Businger, Atmospheric Physics. Blackbody radiation is isotropic. When B λ (T ) is plotted as a function of wavelength on a linear scale the resulting spectrum of monochromatic intensity exhibits the shape illustrated as shown next. 2
10 Blackbody emission (the Planck function) for absolute temperatures as indicated, plotted as a function of wavelength on a linear scale. 3
11 Wien s Displacement Law 4
12 Wien s Displacement Law Differentiating Planck s function and setting the derivative equal to zero yields the wavelength of peak emission for a blackbody at temperature T λ m 2900 T where λ m is expressed in microns and T in degrees kelvin. 4
13 Wien s Displacement Law Differentiating Planck s function and setting the derivative equal to zero yields the wavelength of peak emission for a blackbody at temperature T λ m 2900 T where λ m is expressed in microns and T in degrees kelvin. This equation is known as Wien s Displacement Law. 4
14 Wien s Displacement Law Differentiating Planck s function and setting the derivative equal to zero yields the wavelength of peak emission for a blackbody at temperature T λ m 2900 T where λ m is expressed in microns and T in degrees kelvin. This equation is known as Wien s Displacement Law. On the basis of this equation, it is possible to estimate the temperature of a radiation source from a knowledge of its emission spectrum, as illustrated in an example below. 4
15 Exercise: Prove Wien s Displacement Law. 5
16 Exercise: Prove Wien s Displacement Law. Solution: Planck s function is c B λ = 1 λ 5 exp(c 2 /λt ) 1 5
17 Exercise: Prove Wien s Displacement Law. Solution: Planck s function is c B λ = 1 λ 5 exp(c 2 /λt ) 1 For values of interest in atmospheric and solar science, the exponential term is much larger than unity. Assuming this, we may write B λ = c 1 λ 5 exp(c 2 /λt ) = c 1 λ 5 exp( c 2 /λt ) 5
18 Exercise: Prove Wien s Displacement Law. Solution: Planck s function is c B λ = 1 λ 5 exp(c 2 /λt ) 1 For values of interest in atmospheric and solar science, the exponential term is much larger than unity. Assuming this, we may write B λ = Then, taking logarithms, c 1 λ 5 exp(c 2 /λt ) = c 1 λ 5 exp( c 2 /λt ) log B λ = log c 1 5 log λ c 2 λt 5
19 Exercise: Prove Wien s Displacement Law. Solution: Planck s function is c B λ = 1 λ 5 exp(c 2 /λt ) 1 For values of interest in atmospheric and solar science, the exponential term is much larger than unity. Assuming this, we may write B λ = Then, taking logarithms, c 1 λ 5 exp(c 2 /λt ) = c 1 λ 5 exp( c 2 /λt ) log B λ = log c 1 5 log λ c 2 λt At the maximum, we have db λ dλ = 0 or d log B λ dλ = 0 5
20 We differentiate log B λ = log c 1 5 log λ c 2 λt 6
21 We differentiate Then log B λ = log c 1 5 log λ c 2 λt 5 λ + c 2 λ 2 T = 0 or 5 = c 2 λt or λ = 1 5 c 2 T 6
22 We differentiate Then log B λ = log c 1 5 log λ c 2 λt 5 λ + c 2 λ 2 T = 0 or 5 = c 2 λt or λ = 1 5 c 2 T Since c 2 = m K, we have c 2 / , so λ = (metres) or λ = T T (µm) 6
23 We differentiate Then log B λ = log c 1 5 log λ c 2 λt 5 λ + c 2 λ 2 T = 0 or 5 = c 2 λt or λ = 1 5 c 2 T Since c 2 = m K, we have c 2 / , so λ = (metres) or λ = T T (µm) MatLab Exercise: Plot B λ as a function of λ for T = 300 and T = Use the range λ (0.1 µm, 100 µm). 6
24 We differentiate Then log B λ = log c 1 5 log λ c 2 λt 5 λ + c 2 λ 2 T = 0 or 5 = c 2 λt or λ = 1 5 c 2 T Since c 2 = m K, we have c 2 / , so λ = (metres) or λ = T T (µm) MatLab Exercise: Plot B λ as a function of λ for T = 300 and T = Use the range λ (0.1 µm, 100 µm). Plot B λ for T = 300 and also the approximation obtained by assuming exp(c 2 /λt ) 1 (as used above). 6
25 Exercise: Use Wien s displacement to compute the colour temperature of the sun. 7
26 Exercise: Use Wien s displacement to compute the colour temperature of the sun. Solution: The wavelength of maximum solar emission is observed to be approximately µm. 7
27 Exercise: Use Wien s displacement to compute the colour temperature of the sun. Solution: The wavelength of maximum solar emission is observed to be approximately µm. Hence T = 2900 λ m = = 6100 K 7
28 Exercise: Use Wien s displacement to compute the colour temperature of the sun. Solution: The wavelength of maximum solar emission is observed to be approximately µm. Hence T = 2900 λ m = = 6100 K Wien s displacement law explains why solar radiation is concentrated in the UV, visible and near infrared regions of the spectrum, while radiation emitted by planets and their atmospheres is largely confined to the infrared, as shown in the following figure. 7
29 8
30 Key to above figure (a) Blackbody spectra representative of the sun (left) and the earth (right). The wavelength scale is logarithmic rather than linear, and the ordinate has been multiplied by wavelength in order to make area under the curve proportional to intensity. The intensity scale for the right hand curve has been stretched to make the areas under the two curves the same. 9
31 Key to above figure (a) Blackbody spectra representative of the sun (left) and the earth (right). The wavelength scale is logarithmic rather than linear, and the ordinate has been multiplied by wavelength in order to make area under the curve proportional to intensity. The intensity scale for the right hand curve has been stretched to make the areas under the two curves the same. (c) the atmospheric absorptivity(for flux density) for parallel mean solar (λ < 4 µm) radiation for a solar zenith angle of 50 and isotropic terrestrial ((λ > 4 µm) radiation. 9
32 Key to above figure (a) Blackbody spectra representative of the sun (left) and the earth (right). The wavelength scale is logarithmic rather than linear, and the ordinate has been multiplied by wavelength in order to make area under the curve proportional to intensity. The intensity scale for the right hand curve has been stretched to make the areas under the two curves the same. (c) the atmospheric absorptivity(for flux density) for parallel mean solar (λ < 4 µm) radiation for a solar zenith angle of 50 and isotropic terrestrial ((λ > 4 µm) radiation. (b) as in (c) but for the upper atmosphere defined as levels above 10 km. 9
33 The Stefan-Boltzmann Law 10
34 The Stefan-Boltzmann Law The blackbody flux density obtained by integrating the Planck function B λ over all wavelengths, is given by F = σt 4 where σ is a constant equal to W m 2 K 4. 10
35 The Stefan-Boltzmann Law The blackbody flux density obtained by integrating the Planck function B λ over all wavelengths, is given by F = σt 4 where σ is a constant equal to W m 2 K 4. If a surface emits radiation with a known flux density, this equation can be solved for its equivalent blackbody temperature, that is, the temperature a blackbody would need to have in order to emit the same flux density of radiation. 10
36 The Stefan-Boltzmann Law The blackbody flux density obtained by integrating the Planck function B λ over all wavelengths, is given by F = σt 4 where σ is a constant equal to W m 2 K 4. If a surface emits radiation with a known flux density, this equation can be solved for its equivalent blackbody temperature, that is, the temperature a blackbody would need to have in order to emit the same flux density of radiation. If the surface emits as a blackbody, its actual temperature and its equivalent blackbody temperature will be the same. 10
37 The Stefan-Boltzmann Law The blackbody flux density obtained by integrating the Planck function B λ over all wavelengths, is given by F = σt 4 where σ is a constant equal to W m 2 K 4. If a surface emits radiation with a known flux density, this equation can be solved for its equivalent blackbody temperature, that is, the temperature a blackbody would need to have in order to emit the same flux density of radiation. If the surface emits as a blackbody, its actual temperature and its equivalent blackbody temperature will be the same. Applications of the Stefan Boltzmann Law and the concept of equivalent blackbody temperature are illustrated in the following problems. 10
38 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. 11
39 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. 11
40 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. The Earth-Sun distance is d = m 11
41 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. The Earth-Sun distance is d = m The radius of the solar photosphere is m. 11
42 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. The Earth-Sun distance is d = m The radius of the solar photosphere is m. Solution: We first calculate the flux density at the top of the layer, making use of the inverse square law: F photosphere = F earth ( ) 2 Rsun d 11
43 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. The Earth-Sun distance is d = m The radius of the solar photosphere is m. Solution: We first calculate the flux density at the top of the layer, making use of the inverse square law: F photosphere = F earth ( ) 2 Rsun d Therefore F photosphere = ( ) = W m 2 11
44 Exercise. Calculate the equivalent blackbody temperature of the solar photosphere, the outermost visible layer of the sun, based on the following information. The flux density of solar radiation reaching the earth is 1370 W m 2. The Earth-Sun distance is d = m The radius of the solar photosphere is m. Solution: We first calculate the flux density at the top of the layer, making use of the inverse square law: Therefore F photosphere = F earth F photosphere = ( ) 2 Rsun From the Stefan-Boltzmann Law, we get ( d ) = W m 2 σt 4 E = W m 2 11
45 Again, from the Stefan-Boltzmann Law, we get σt 4 E = W m 2 12
46 Again, from the Stefan-Boltzmann Law, we get σt 4 E = W m 2 So, the equivalent temperature is ( ) /4 T E = = 4 ( ) = 5770 K 12
47 Again, from the Stefan-Boltzmann Law, we get σt 4 E = W m 2 So, the equivalent temperature is ( ) /4 T E = = 4 ( ) = 5770 K That this value is slighly lower than the sun s colour temperature estimated in the previous exercise is evidence that the spectrum of the sun s emission differs slighly from the blackbody spectrum prescribed by Planck s law. 12
48 Exercise: Calculate the equivalent blackbody temperature of the earth assuming a planetary albedo of Assume that the earth is in radiative equilibrium with the sun: i.e., that there is no net energy gain or loss due to radiative transfer. 13
49 Exercise: Calculate the equivalent blackbody temperature of the earth assuming a planetary albedo of Assume that the earth is in radiative equilibrium with the sun: i.e., that there is no net energy gain or loss due to radiative transfer. Solution: Let F S be the flux density of solar radiation incident upon the earth (1370 W m 2 ); 13
50 Exercise: Calculate the equivalent blackbody temperature of the earth assuming a planetary albedo of Assume that the earth is in radiative equilibrium with the sun: i.e., that there is no net energy gain or loss due to radiative transfer. Solution: Let F S be the flux density of solar radiation incident upon the earth (1370 W m 2 ); F E the flux density of longwave radiation emitted by the earth, 13
51 Exercise: Calculate the equivalent blackbody temperature of the earth assuming a planetary albedo of Assume that the earth is in radiative equilibrium with the sun: i.e., that there is no net energy gain or loss due to radiative transfer. Solution: Let F S be the flux density of solar radiation incident upon the earth (1370 W m 2 ); F E the flux density of longwave radiation emitted by the earth, R E the radius of the earth, 13
52 Exercise: Calculate the equivalent blackbody temperature of the earth assuming a planetary albedo of Assume that the earth is in radiative equilibrium with the sun: i.e., that there is no net energy gain or loss due to radiative transfer. Solution: Let F S be the flux density of solar radiation incident upon the earth (1370 W m 2 ); F E the flux density of longwave radiation emitted by the earth, R E the radius of the earth, and A the planetary albedo of the earth. 13
53 Calculate the earth s equivalent blackbody temperature T E : F E = σt 4 E = (1 A)F S 4 = = 240 W m 2 14
54 Calculate the earth s equivalent blackbody temperature T E : F E = σt 4 E = (1 A)F S 4 = = 240 W m 2 14
55 Calculate the earth s equivalent blackbody temperature T E : F E = σt 4 E = (1 A)F S 4 = = 240 W m 2 Solving for T E, we obtain T E = 4 F E /σ = 255 K = 18 C 14
56 Equivalent blackbody temperature of some of the planets, based on the assumption that they are in radiative equilibrium with the sun. Planet Dist. from sun Albedo TE (K) Mercury 0.39 AU Venus 0.72 AU Earth 1.00 AU Mars 1.52 AU Jupiter 5.18 AU
57 Kirchhoff s Law 16
58 Kirchhoff s Law Gaseous media are not blackbodies, but their behavior can nonetheless be understood byapplying the radiation laws derived for blackbodies. For this purpose it is useful to define the emissivity and the absorptivity of a body 16
59 Kirchhoff s Law Gaseous media are not blackbodies, but their behavior can nonetheless be understood byapplying the radiation laws derived for blackbodies. For this purpose it is useful to define the emissivity and the absorptivity of a body The emissivity is the ratio of the monochromatic intensity of the radiation emitted by the body to the corresponding blackbody radiation ε λ = I λ(emitted) B λ (T ) 16
60 Kirchhoff s Law Gaseous media are not blackbodies, but their behavior can nonetheless be understood byapplying the radiation laws derived for blackbodies. For this purpose it is useful to define the emissivity and the absorptivity of a body The emissivity is the ratio of the monochromatic intensity of the radiation emitted by the body to the corresponding blackbody radiation ε λ = I λ(emitted) B λ (T ) The absorptivity is the fraction of the incident monochromatic intensity that is absorbed A λ = I λ(absorbed) I λ (incident) 16
61 Kirchhoff s Law states that under conditions of thermodynamic equilibrium ε λ = A λ 17
62 Kirchhoff s Law states that under conditions of thermodynamic equilibrium ε λ = A λ Kirchhoff s Law implied that a body which is a good absorber of energy at a particular wavelength is also a good emitter at that wavelength. 17
63 Kirchhoff s Law states that under conditions of thermodynamic equilibrium ε λ = A λ Kirchhoff s Law implied that a body which is a good absorber of energy at a particular wavelength is also a good emitter at that wavelength. Likewise, a body which is a poor absorber at a given wavelength is also a poor emitter at that wavelength. 17
64 End of
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