chaPtEr 1
:
BlackBody
radiation
SCOPE OF STUDY
Concept of black body

SUB TOPICS

Stefan’s Law, energy

Wien’s displacement

spectrum

law
introduction
The black body notion is important in studying thermal radiation and
electromagnetic radiation energy transfe...
Concept of black body
Black Body
Black Body

An ideal body which absorbs all the electromagnetic
radiation that strikes it...
Concept of black body
Why black body??
Because those bodies that absorb incident visible light well seem
black to the huma...
Concept of black body
Application :
Optical band (surfaces approach an ideal black body in their
ability to absorb radiati...
Concept of black body
Concept of black body
Black body radiation //
Black body radiation
Cavity radiation
Cavity radiation

The electromagnetic ...
Concept of black body
Where are the black body radiation comes from??
Sources of black body radiation :
Cosmic microwave b...
Concept of black body
The Sun


the presence of thermal black body radiation with a brightness

temperature of 5800K at t...
Concept of black body
Blackbody radiation is emitted as a broad spectrum of wavelengths

Figure ::The characteristic graph...
Energy spectrum
EM Radiation : A kind of radiation including visible light, radio
waves, gamma rays, and X-rays, in which ...
Energy spectrum
Energy spectrum
Spectrum of Electromagnetic Radiation
Region

Wavelength
(Angstroms)

Wavelength
(centimeters)

Frequency
...
black body RADIATION
LAWS

Laws

Stefan’s
Law

Wein’s
Displacement
Law
Stefan’s law
Stefan’s Law or Stefan’s Boltzmann’s Law
Stefan’s Law or Stefan’s Boltzmann’s Law

The energy radiated by a b...
Stefan’s law
Formula

where

P = Energy/ time = Power
A = Area
T = Temperature
σ = Stefan-Boltzmann constant
Stefan’s law
Stefan’s Law (1879, 1884)
 Josef Stefan deduced the rule in 1879 and Ludwig Boltzmann
provided a formal deri...
Stefan’s law
For hot objects other than ideal radiators, the law is expressed in the form:

where e is the emissivity of t...
Stefan’s law
If the hot object is radiating energy to its cooler surroundings at
temperature Tc, the net radiation loss ra...
wein’s displacement law
Wein’s
Wein’s
Displacement Law,
Displacement Law,
1893
1893

 The wavelength distribution peaks at...
wein’s displacement law
Formula
Formula

λmax = c = 2.898x10-3
λmax = c = 2.898x10-3
T
T
T

Unit constant, c : meter per K...
wein’s displacement law
Wien's Law tells us that objects of different temperature emit spectra
that peak at different wave...
wein’s displacement law
wein’s displacement law

Temperature , T (

), Radiated energy, E ( ), Wavelength, λ ( )
wein’s displacement law

Black body thermal emission intensity as a function of wavelength
for various (absolute) temperat...
wein’s displacement law
Examples:
•Light from the Sun and Moon. The surface temperature (or more correctly, the
effective ...
wein’s displacement law
•Radiation from mammals and the living human body. Mammals at roughly 300 K
emit peak radiation at...
~ ~The end~ ~
“If you really want to
do somethIng, you wIll
fInd a way. If you don't,
you wIll fInd an
excuse.“
-JIm rohn-
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Chapter 1 blackbody radiation

  1. 1. chaPtEr 1 : BlackBody radiation
  2. 2. SCOPE OF STUDY Concept of black body SUB TOPICS Stefan’s Law, energy Wien’s displacement spectrum law
  3. 3. introduction The black body notion is important in studying thermal radiation and electromagnetic radiation energy transfer in all wavelength bands. Black body as an ideal radiation absorber and it is used as a standard for comparison with the radiation of real physical bodies. This notion and its characteristics are sometimes are used in describing and studying artificial, quasi deterministic electromagnetic radiation (in radio and TV- broadcasting and communication).
  4. 4. Concept of black body Black Body Black Body An ideal body which absorbs all the electromagnetic radiation that strikes it so that all incident radiation is completely absorbed.
  5. 5. Concept of black body Why black body?? Because those bodies that absorb incident visible light well seem black to the human eye. Example: We can hardly characterize our sun which is indeed almost a black body within a very wide band of electromagnetic radiation wavelength as a black physical object in optics. It is namely bright-white sunlight which represents the equilibrium black body radiation.
  6. 6. Concept of black body Application : Optical band (surfaces approach an ideal black body in their ability to absorb radiation) such as soot, silicon carbide, platinum and golden niellos. Earth surfaces (water surfaces, ice, land) absorb infrared radiation well and in thermal IR band, these physical objects are ideal black bodies.
  7. 7. Concept of black body
  8. 8. Concept of black body Black body radiation // Black body radiation Cavity radiation Cavity radiation The electromagnetic radiation that would be radiated from an ideal black body
  9. 9. Concept of black body Where are the black body radiation comes from?? Sources of black body radiation : Cosmic microwave background (CMB) of the universe – fluctuation electromagnetic radiation that fills the part of the universe.  the radiation possesses nearly isotropic spatial-angular field with an intensity that can be characterized by the radio brightness temperature of 2.73K.  to determine accuracy, direction and velocity of motion of the solar system.  as a re-reflected radiation to investigate the emissive characteristics of terrestrial surfaces.
  10. 10. Concept of black body The Sun  the presence of thermal black body radiation with a brightness temperature of 5800K at the Sun.  along with a black body radiation, there exist powerful, non- stationary quasi-noise radiation (flares, storms). The Earth  possesses radiation close to black body radiation with a thermodynamic temperature of 287K.
  11. 11. Concept of black body Blackbody radiation is emitted as a broad spectrum of wavelengths Figure ::The characteristic graph of the thermal radiation emitted by Figure The characteristic graph of the thermal radiation emitted by aahot object hot object
  12. 12. Energy spectrum EM Radiation : A kind of radiation including visible light, radio waves, gamma rays, and X-rays, in which electric and magnetic fields vary simultaneously. Energy spectrum based on the EM spectrum. EM Spectrum : The distribution of electromagnetic radiation according to energy (or equivalently, by virtue of the relations in the previous section, according to frequency or wavelength).
  13. 13. Energy spectrum
  14. 14. Energy spectrum Spectrum of Electromagnetic Radiation Region Wavelength (Angstroms) Wavelength (centimeters) Frequency (Hz) Energy (eV) Radio > 109 > 10 < 3 x 109 < 10-5 Microwave 109 - 106 10 - 0.01 3 x 109 - 3 x 1012 10-5 - 0.01 Infrared 106 - 7000 0.01 - 7 x 10-5 3 x 1012 - 4.3 x 1014 0.01 - 2 Visible 7000 - 4000 Ultraviolet 4000 - 10 4 x 10-5 - 10-7 7.5 x 1014 - 3 x 1017 3 - 103 X-Rays 10 - 0.1 10-7 - 10-9 3 x 1017 - 3 x 1019 103 - 105 Gamma Rays < 0.1 < 10-9 > 3 x 1019 > 105 7 x 10-5 - 4 x 10-5 4.3 x 1014 - 7.5 x 1014 2-3
  15. 15. black body RADIATION LAWS Laws Stefan’s Law Wein’s Displacement Law
  16. 16. Stefan’s law Stefan’s Law or Stefan’s Boltzmann’s Law Stefan’s Law or Stefan’s Boltzmann’s Law The energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature.
  17. 17. Stefan’s law Formula where P = Energy/ time = Power A = Area T = Temperature σ = Stefan-Boltzmann constant
  18. 18. Stefan’s law Stefan’s Law (1879, 1884)  Josef Stefan deduced the rule in 1879 and Ludwig Boltzmann provided a formal derivation in 1884.  Classical physics  Explain the growth in the height of the curve as the temperature increase.  Energy emitted increase rapidly with an increase in temperature which is proportional to the temperature raised to the fourth power.
  19. 19. Stefan’s law For hot objects other than ideal radiators, the law is expressed in the form: where e is the emissivity of the object (e = 1 for ideal radiator). e = characteristic of the surface of the radiating material ( 0 < e < 1) black surface such as charcoal, e close to 1, shinny metal surfaces have e close to 0 (emit less radiation and absorb little radiation that falls upon them). e depends on the temperature of material. Black and very dark object is good emitter and good absorber. Example : The light-colored clothing is preferable to dark clothing on a hot day.
  20. 20. Stefan’s law If the hot object is radiating energy to its cooler surroundings at temperature Tc, the net radiation loss rate takes the form The above equation is valid for T = T1 = temperature of the surface area of the object and Tc = T2 = Temperature of surrounding
  21. 21. wein’s displacement law Wein’s Wein’s Displacement Law, Displacement Law, 1893 1893  The wavelength distribution peaks at a  The wavelength distribution peaks at a value that is inversely proportional to the value that is inversely proportional to the temperature. temperature.
  22. 22. wein’s displacement law Formula Formula λmax = c = 2.898x10-3 λmax = c = 2.898x10-3 T T T Unit constant, c : meter per Kelvin (m/K) The ratio of the maximum wavelengths for two temperatures, T and T',
  23. 23. wein’s displacement law Wien's Law tells us that objects of different temperature emit spectra that peak at different wavelengths. Hotter objects emit most of their radiation at shorter wavelengths, hence they will appear to be bluer . Cooler objects emit most of their radiation at longer wavelengths, hence they will appear to be redder. Furthermore, at any wavelength, a hotter object radiates more (is more luminous) than a cooler one.
  24. 24. wein’s displacement law
  25. 25. wein’s displacement law Temperature , T ( ), Radiated energy, E ( ), Wavelength, λ ( )
  26. 26. wein’s displacement law Black body thermal emission intensity as a function of wavelength for various (absolute) temperatures.
  27. 27. wein’s displacement law Examples: •Light from the Sun and Moon. The surface temperature (or more correctly, the effective temperature) of the Sun is 5778 K. Using Wien's law, this temperature corresponds to a peak emission at a wavelength of 2.90 × 10 6 nm-K / 5778 K = 502 nm = about 5000 Å. This wavelength is (not incidentally) fairly in the middle of the most sensitive part of land animal visual spectrum acuity. •Light from incandescent bulbs and fires. A lightbulb has a glowing wire with a somewhat lower temperature, resulting in yellow light, and something that is "red hot" is again a little less hot. It is easy to calculate that a wood fire at 1500 K puts out peak radiation at 2.90 × 106 nm-K / 1500 K = 1900 nm = 19,000 Å. This is far more energy in the infrared than in the visible band, which ends about 7500 Å.
  28. 28. wein’s displacement law •Radiation from mammals and the living human body. Mammals at roughly 300 K emit peak radiation at 2900 μm-K / 300 K ~ 10 μm, in the far infrared. This is, therefore, the range of infrared wavelengths that pit viper snakes and passive IR cameras must sense. •The wavelength of radiation from the Big Bang. A typical application of Wien's law would also be to the blackbody radiation resulting from the Big Bang. Remembering that Wien's displacement constant is about 3 mm-K, and the temperature of the Big Bang background radiation is about 3 K (actually 2.7 K), it is apparent that the microwave background of the sky peaks in power at 2.9 mm-K / 2.7 K = just over 1 mm wavelength in the microwave spectrum. This provides a convenient rule of thumb for why microwave equipment must be sensitive on both sides of this frequency band, in order to do effective research on the cosmic microwave background.
  29. 29. ~ ~The end~ ~ “If you really want to do somethIng, you wIll fInd a way. If you don't, you wIll fInd an excuse.“ -JIm rohn-
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