1. VISHWAKARMA GOVERNMENT ENGINEERING COLLEGE
NAME : MORI NIKUNJ J.
ENROLLMENT NO. :
BRANCH : MECHANICAL
SUBJECT : HEAT TRANSFER

2. BLACKBODY AND GRAYBODY RADIATION

3. What happens to this radiation?
What is a black-body?
An object that absorbs all incident radiation, i.e. no reflection
A small hole cut into a cavity is the most popular and realistic example.
None of the incident radiation escapes
What happens to this radiation?
The radiation is absorbed in the walls of the cavity
This causes a heating of the cavity walls
Atoms in the walls of the cavity will vibrate at frequencies characteristic of the temperature of the walls
These atoms then re-radiate the energy at this new characteristic frequency

4. The emitted "thermal" radiation characterizes the equilibrium temperature of the black-body

5. Black-body spectrum

6. Black-body spectrum
Black-bodies do not "reflect" any incident radiation
They may re-radiate, but the emission characterizes the black-body only
The emission from a black-body depends only on its temperature
We (at 300 K) radiate in the infrared
Objects at K start to glow
At high T, objects may become white hot
Found empirically by Joseph Stefan (1879); later calculated by Boltzmann
s = × 10-8 W.m-2.K-4.
A black-body reaches thermal equilibrium when the incident radiation power is balanced by the power re-radiated, i.e. if you expose a black-body to radiation, its temperature rises until the incident and radiated powers balance.
Stefan-Boltzmann Law
Power per unit area radiated by black-body R = s T 4
Wien's displacement Law
lm T = constant = × 10-3 m.K, or lm  T-1

7. Gray body A gray body is define as a body whose absorptivuty of a surface does not very with variation in temperature and wavelength of the incident radiation.

8. White body
If all incident radiation fall on the body are reflected is called a white body.

9. THANK YOU