Reflection, Transmission and Absorption of Heat radiations
When heat falls on a surface, some part of it is absorbed, a part of it is transmitted and some part of it is reflected. Suppose Q is the total heat energy falling in the body, then R is the reflected, T is the transmitted and A is the Absorbed heat energy as shown in the figure.
R+A+T = QDividing both sides by Q,
R/Q + A/Q + T/Q = Q/Q
r+ a + t =1 ……. (i)
where,
r = R/Q, reflectance or reflection coefficient
a = A/Q, absorptance or absorption coefficient
t = T/Q, transmittance or transmission coefficient
Black body and black body radiations
A perfect black body is one which absorbs heat radiation of all wavelengths that fall upon it. The black body neither reflects nor transmits the incident heat radiation and hence the body appears black. But when it is heated, it loses all the heat and emits the radiation of all wavelengths which is called black body radiation. Till now, a perfect black body has not been discovered yet but still, lampblack absorbs 96% to 98% of the heat which is a perfect black body for practical use.
Ferry’s Black body
Ferry developed a black body as shown in the figure which consists of a double-walled metallic sphere having a small opening on one side and a small projection on the other side. It is coated with lampblack in the inner part and a heating coil is placed between the walls and the radiations falling into the body are reflected many times and after some time almost every radiation is absorbed. When heated the body emits all the radiation through the opening.
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