2. SCOPE OF STUDY
Concept of black body
SUB TOPICS
Stefan’s Law, energy
Wien’s displacement
spectrum
law
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. 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. 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. 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.
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. 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. 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. 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. 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).
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.
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. 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. 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. 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. 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. 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.
26. wein’s displacement law
Black body thermal emission intensity as a function of wavelength
for various (absolute) temperatures.
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. 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. ~ ~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-
