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Radiations: Heat transfer by electromagnetic waves. Also called Heat waves. No medium is required for their propagation.  Example,  Radiatons  (ultraviolet , infrared ,visible) radiations from Sun to Earth.

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Theory of exchange : All body emits radiations at all Temperature. Amount of radiations emitted / absorbed depends on Temperature and nature of material. All body absorb radiations falling on it at all Temperature. When a body absorb radiations its temperature increases, when it emit radiations its temperature decreases .

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Black body: 	 A body that absorbs all radiations falling on it at any Temperature, at any incident angle. Having absorptive power a = 1 Black body is ideal body A perfectly black body has zero reflection , zero transmittance Black body need not to be black in color. It may be red , blue or yellow. Black body color depends on temperature.

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Kirchoff’s  law of radiation: According to  Kirchoff’s  law                “Good absorber is a good emitter.” Example of practical  black body are Sun/Star Lamp black (1% reflect) Graphite (3% reflect) Platinum black Gold black Isothermal enclosure

Black Body Radiations

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