Ferry,s Black Body

 Perfectly black body

Introduction : As we already studied that total heat energy incident on a body is sum of radiation absorbed by the body, radiation reflected by the body and radiation transmitted by the body. Thus accordingly we arrive a equation Q= Q_a + Q_r =Q_t . Thus based on that a material can be good absorber, reflector and transmitter and material can be poor absorber , reflector , transmitter.

Let us consider an example below using an illustrative experiment 

Consider two body A and B exactly of same area and same temperature initially. Body A is polished with shiny silver and body B is polished with dark black as seen in the picture above. Both the system is  exposed to thermal radiation. Accordingly some part of heat is absorbed, reflected and transmitted by both the body depending on their respective coefficient of  absorption, reflection, transmission.

The above experiment conclude that the body which is polished with shiny material absorb very less heat and the body which is black painted absorb more heat. To maintain their temperature the rate of absorbed heat is always equal to rate of emission. Thus the body A which is poor absorber is also a poor emitter but good reflector. On the other hand the body which is good absorber is good emitter and poor reflector. 

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Ferry's Black Body

It is a double walled structured. Outer surface is polished with silver and inner surface is polished with lamp black. It has a small aperture for incident of radiation inside the body. There is a small conical projection in front of aperture to avoid reflection back of radiation via the same path. Thus the radiation entering the body cannot escape out and thus about 98% of energy is absorbed.

<<< Specific heat capacity>>> l <<<Radiation spectrum>>>

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