Absorptivity Reflectivity Transmissivity Emissivity Grey Body Black Body Laws of black body radiation Stefan-Boltzmann law Planck’s Law Wiens Displacement law Conclusion

1. Process Heat Transfer
(2140503)
TOPIC
Concept of Grey body and Black body
Presented by
Paranami Chintan (160170105043)
Rajput Mayank (160170105045)
Rana Pranav (160170105046)

2. CONTAINS
 Basic Terms
 Absorptivity
 Reflectivity
 Transmissivity
 Emissivity
 Grey Body
 Black Body
 Laws of black body radiation
Stefan-Boltzmann law
Planck’s Law
Wiens Displacement law
 Conclusion

3. Before the understanding concept of
grey body and black body we should
understand some terms like
Absorptivity
Reflectivity
Transmissivity
Emissivity
Monochromic Emissivity

4. The Fraction of incident
radiation on a body that is
ABSORBED by the body is known
as the ABSORPTIVITY .
It may ne denoted by the letter
“a” .
The Fraction of incident
radiation on a body that is
REFLECTED by the body is known
as the REFLECTIVITY .
It may ne denoted by the letter
“r” .
The Fraction of incident
radiation on a body that is
TRANSMITTED by the body is
known as the TRANSMISSIVITY .
It may ne denoted by the letter
“τ ” .
It is define as the ratio of
emissive power E of the body to
that of a black body Eb at the
same temperature.
It may be denoted by the letter
“e” .
ABSORPTIVITY REFLECTIVITY
TRANSMISSIVITY EMISSIVITY

5. GREY BODY
 A body having the
same value of the
monochromatic
emissivity at all the
wave lengths is called
grey body.
 A Grey body is the one
of which emissivity is
independent of
wavelength.
MONOCHOMATIC
EMISSIVITY
It is the ratio of the
monochromatic
emissive power of a
surface to that of a
black surface at the
same wavelength.


6. BLACK BODY
A body for which a=1, r=t= 0, i.e which absorbs
all the incident radiant energy, is called a black
body.
It neither reflects nor transmits but absorbs all
the radiation incident on it, so it treated as ideal
radiation receiver.
It is not necessary that surface is black in colour .
It absorbs 96% of visible light
Both absorptivity and emissivity of a perfectly
black body are unity.

7. DEFINATION OF 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 property is
valid for all wavelengths and to
all angels of incidence.
Therefore, the blackbody is an
ideal absorber of incident
radiation.

8. BLACK BODY RADIATION LAW
1. Stefan-Boltzmann law:
 It states that the total emissive power of a black
body is directly proportional to the fourth power of
its absolute temperature.
Eb α T4
Eb = σ T4
Where T = Temperature in K
σ = Stefan-Boltzmann constant
= 5.67 * 10-8 W/m2.K4

9. 2. Planck’s Law
This law gives a relationship between the
monochromatic emissive power of a black body,
absolute temperature and the corresponding
wavelength.
Eb, λ = -------
Where : Eb,λ = monochromatic emissive power of the body surface,
W/(m2μm). h= Planck's constant (6.625 * 10-3)
K = Boltzmann constant C = speed of light
T = absolute Temperature λ = wavelength
2πhc2λ-5
e hc/kλT -1

10. 3. Wiens Displacement law:
 It states that the wavelength at which the maximum
monochromatic emissive power is obtained in inversely
proportional to the absolute temp., or
T λmax = C
Where : λmax = micrometers
T = temp in K
C = constant (2890)
 This law gives a relationship between the wavelength at
which maximum emissive power is attained and the
absolute temperature.

11. CONCLUSION
As the temperature increase , the peak wavelength emitted
by the black body decreases.
As temperature increases, the total energy emitted
increases , because the total area under the curve
increases.
The curve gets infinitely close to the x- axis but never
touches it.