Black Body

Perfectly Black Body :- Perfectly black body is a body which absorbs all the radiant energy incident on it.

For perfectly black body,

1.  Coefficient of absorption is one.

2.  Coefficient of reflection and coefficient of transmission both are equal to zero.

         A hot black body emits more energy than the cold black body. Perfectly black body does not exists in nature. However Lampblack absorbs 98% of the radiant heat incident upon it. So, for the all practical purpose lamp black is treated as perfectly black body.

Ferry’s Black body :-

         Ferry’s black body is an artificial black body.

It consist of,

1.  A double walled hollow metal sphere having small aperture.

2.  The inner surface of lampblack is coated with lampblack, outer surface is nickel polished and it has conical projection which is diametrically opposite to the aperture.

3.  The space between the two walls is evacuated, to avoid the loss of heat by conduction and convection.   

Working :

         When the radiant energy enters through a small aperture. the radiant heat entering the sphere suffers multiple internal reflections. Whenever the radiant heat falls on lampblack, about 98% of incident radiant heat is absorbed. Hence after a large number of reflections, almost all the radiant heat is absorbs by the sphere. Thus the aperture acts as a perfectly black body.