Blackbody ppt

1. Mr.S.SelvakumarMr.S.Selvakumar
Asst professorAsst professor
Department of PhysicsDepartment of Physics
Thamirabharani Engineering Thamirabharani Engineering
CollegeCollege

2. 22
ContentsContents

MotivationMotivation
 Definition of a Black-BodyDefinition of a Black-Body
 Black-Body Radation LawsBlack-Body Radation Laws
1- The Planck Law 1- The Planck Law
2- The Wien Displacement Law2- The Wien Displacement Law
3- The Stefan-Boltzmann Law3- The Stefan-Boltzmann Law
4- The Rayleigh-Jeans Law4- The Rayleigh-Jeans Law
Application for Black Body Application for Black Body
ConclusionConclusion
SummarySummary

3. 33
MotivationMotivation
•The black body is importance in thermal The black body is importance in thermal
radiation theory and practice. radiation theory and practice.
•The ideal black body notion is importance The ideal black body notion is importance
in studying thermal radiation and in studying thermal radiation and
electromagnetic radiation transfer in all electromagnetic radiation transfer in all
wavelength bands. wavelength bands.
•The black body is used as a standard with The black body is used as a standard with
which the absorption of real bodies is which the absorption of real bodies is
compared.compared.

4. 44
Definition of a black bodyDefinition of a black body
A black body is an ideal body
which allows the whole of the
incident radiation to pass into
itself ( without reflecting the
energy ) and absorbs within
itself this whole incident
radiation (without passing on
the energy). This propety is
valid for radiation corresponding
to all wavelengths and to all
angels of incidence. Therefore,
the black body is an ideal
absorber of incident radaition.
Univ. of Oregon web site

5. 55
Black-Body Radiation Laws (1)Black-Body Radiation Laws (1)
1- The Rayleigh-Jeans Law1- The Rayleigh-Jeans Law..
* It agrees with experimental * It agrees with experimental
measurements for long measurements for long
wavelengths.wavelengths.
* It predicts an energy output that * It predicts an energy output that
diverges towards infinity as diverges towards infinity as
wavelengths grow smaller.wavelengths grow smaller.
* The failure has become known * The failure has become known
as the ultraviolet catastrophe.as the ultraviolet catastrophe.
4
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ckT
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http://www.egglescliffe.org.uk/physics/astronomy/bl
ackbody/Image22c.gif

6. 66
This formula also had a This formula also had a
problem. The problem was problem. The problem was
the term in the the term in the
denominator.denominator.
For large wavelengths it For large wavelengths it
fitted the experimental data fitted the experimental data
but it had major problems at but it had major problems at
shorter wavelengths.shorter wavelengths.
Ultraviolet CatastropheUltraviolet Catastrophe
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http://theory.uwinnipeg.ca/users/gabor/foundations
/quantum/images/slide5.gif

7. 77
2- Planck Law2- Planck Law
-We have two forms. As a function We have two forms. As a function
of wavelength.of wavelength.
And as a function of frequency And as a function of frequency
The Planck Law gives a distribution that The Planck Law gives a distribution that
peaks at a certain wavelength, the peak peaks at a certain wavelength, the peak
shifts to shorter wavelengths for higher shifts to shorter wavelengths for higher
temperatures, and the area under the temperatures, and the area under the
curve grows rapidly with increasing curve grows rapidly with increasing
temperature.temperature.
Black-Body Radiation Laws (2)Black-Body Radiation Laws (2)
http://scienceworld.wolfram.com/physics/PlanckLaw.html
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8. 88
Comparison between Classical and Comparison between Classical and
Quantum viewpointQuantum viewpoint
http://upload.wikimedia.org/wikipedia/commons/a/a1/Blackbody-lg.png
There is a good fit at long wavelengths, but at short wavlengths there is a
major disagreement. Rayleigh-Jeans ∞, but Black-body 0.

9. 99
3- Wein Displacement Law3- Wein Displacement Law
- It tells us as we heat an object up, its - It tells us as we heat an object up, its
color changes from red to orange to color changes from red to orange to
white hotwhite hot..
- You can use this to calculate the - You can use this to calculate the
temperature of stars.temperature of stars.
The surface temperature of the Sun is The surface temperature of the Sun is
5778 K, this temperature 5778 K, this temperature
corresponds to a peak emission = corresponds to a peak emission =
502 nm = about 5000 Å.502 nm = about 5000 Å.
- b is a constant of proportionality, - b is a constant of proportionality,
called Wien's displacement constant called Wien's displacement constant
and equals 2.897 768 5(51) × 10and equals 2.897 768 5(51) × 10
–3–3
m m
K = 2.897768 5(51) × 10K = 2.897768 5(51) × 10
66
nm nm K.K.
Black-Body Radiation Laws (3)Black-Body Radiation Laws (3)
http://www.rumford.com/radiant/images/W
iengraph.gif
T
b
=
maxl

10. 1010
4- The Stefan-Boltzmann Law4- The Stefan-Boltzmann Law
* Gives the total energy being * Gives the total energy being
emitted at all wavelengths by emitted at all wavelengths by
the blackbody (which is the the blackbody (which is the
area under the Planck Law area under the Planck Law
curve).curve).
* Explains the growth in the * Explains the growth in the
height of the curve as the height of the curve as the
temperature increases. Notice temperature increases. Notice
that this growth is very abrupt.that this growth is very abrupt.
* * Sigma = 5.67 * 10Sigma = 5.67 * 10
-8-8
J s J s
-1-1
m m
-2-2
K K
-4-4
, ,
Known as the Stefan-Known as the Stefan-
Boltzmann constant.Boltzmann constant.
Black-Body Radiation Laws (4)Black-Body Radiation Laws (4)
4
Tjs=
http://ceos.cnes.fr:8100/cdrom-98/ceos1/science/dg/fig10.gif

11. 1111
Black-Body Radiation Laws (5)Black-Body Radiation Laws (5)
http://en.wikipedia.org/wiki/Image:
RWP-comparison.svg
Comparison of Rayleigh-Jeans law with Wien's
law and Planck's law, for a body of 8 mK
temperature.

12. 1212
Application for Black BodyApplication for Black Body
- - The area of Earth's disk as viewed from The area of Earth's disk as viewed from
space is, Area = πrspace is, Area = πr
22
..
- The total energy incident on Earth is, - The total energy incident on Earth is,
Incident energy = (πrIncident energy = (πr
22
)S)S
oo..
- The energy absorbed by the - The energy absorbed by the
Earth/atmosphere system, as viewed from Earth/atmosphere system, as viewed from
space isspace is
Absorbed energy = (πrAbsorbed energy = (πr
22
)S)S
oo(1 - A). As we know (1 - A). As we know
that bodies must be in radiative that bodies must be in radiative
equilibrium. The solar energy striking equilibrium. The solar energy striking
Earth's disk as viewed from space is re-Earth's disk as viewed from space is re-
emitted as thermal radiation by the emitted as thermal radiation by the
surface of the entire globe, as described surface of the entire globe, as described
by the Stefan-Boltzmann Law, Emitted by the Stefan-Boltzmann Law, Emitted
energy = (4πrenergy = (4πr
22
)σT)σT
44
..
- Set the absorbed energy equal to the - Set the absorbed energy equal to the
emitted energy:emitted energy:
(πr(πr
22
)S)S
oo(1 - A) = (4πr(1 - A) = (4πr
22
)σT)σT
EE
44
, Solving for T yields:, Solving for T yields:
TT
EE = [S = [S
oo(1 - A)/(4σ)](1 - A)/(4σ)]
(1/4)(1/4)
= [1370•(1-0.3)/(4•5.67x10-= [1370•(1-0.3)/(4•5.67x10-
88
)])]
(1/4)(1/4)
= 255 K. = 255 K.
http://soer.justice.tas.gov.au/2003/image/378/atm_structure/o_atm_Radn_Balance-m.jpg

13. 1313
ConclusionConclusion
As the temperature As the temperature
increases, the peak increases, the peak
wavelength emitted by the wavelength emitted by the
black body decreases.black body decreases.
As temperature increases, As temperature increases,
the total energy emitted the total energy emitted
increases, because the increases, because the
total area under the curve total area under the curve
increases.increases.
The curve gets infinitely The curve gets infinitely
close to the x-axis but close to the x-axis but
never touches it.never touches it.
http://www.astro.ufl.edu/~oliver/ast3722/lectures/BasicDetectors/BlackBody.gif

14. 1414
SummarySummary
- A black body is a theoretical object that absorbs 100% of the - A black body is a theoretical object that absorbs 100% of the
radiation that hits it. Therefore it reflects no radiation and radiation that hits it. Therefore it reflects no radiation and
appears perfectly black.appears perfectly black.
- Roughly we can say that the stars radiate like blackbody - Roughly we can say that the stars radiate like blackbody
radiators. This is important because it means that we can use radiators. This is important because it means that we can use
the theory for blackbody radiators to infer things about stars.the theory for blackbody radiators to infer things about stars.
- At a particular temperature the black body would emit the - At a particular temperature the black body would emit the
maximum amount of energy possible for that temperature.maximum amount of energy possible for that temperature.
- Blackbody radiation does not depend on the type of object - Blackbody radiation does not depend on the type of object
emitting it. Entire spectrum of blackbody radiation depends on emitting it. Entire spectrum of blackbody radiation depends on
only one parameter, the temperature, T.only one parameter, the temperature, T.

15. 1515
Thanks for your Thanks for your
attentionattention
Any suggestions or questions are Any suggestions or questions are
welcome.welcome.