VISHWAKARMA GOVERNMENT ENGINEERING COLLEGE NAME : MORI NIKUNJ J. ENROLLMENT NO. : 130170119021 BRANCH : MECHANICAL SUBJECT : HEAT TRANSFER

BLACKBODY AND GRAYBODY RADIATION

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

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

Black-body spectrum

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 600 - 700 K start to glow At high T, objects may become white hot Found empirically by Joseph Stefan (1879); later calculated by Boltzmann s = 5.6705 × 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 = 2.898 × 10-3 m.K, or lm  T-1

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.

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

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