Perfect blackbody

Perfect blackbody:

A body, which absorbs the entire radiant energy incident on it, is called an ideal or perfect blackbody. Thus, for a perfect blackbody, a = 1. Any surface that absorbs all the energy incident on it, and does not reflect any energy, therefore, appears black (unless its temperature is very high to be self-luminous). Lamp black or platinum black that absorb nearly 97% of incident radiant heat, resemble a perfect blackbody. 

Ferry’s blackbody:

It consists of a double walled hollow sphere having tiny hole or aperture, through which radiant heat can enter. The space between the walls is evacuated and outer surface of the sphere is silvered. The inner surface of sphere is coated with lampblack. There is a conical projection on the inner surface of sphere opposite the aperture. Radiation entering through the small hole has negligible chance of escaping back through the small hole. A heat ray entering the sphere through the aperture suffers multiple reflections and is almost completely absorbed inside. Thus, the aperture behaves like a perfect blackbody. 

Watch these videos to learn more about blackbody radiation:

Emission of heat radiation:

Pierre Prevost published a theory of radiation known as theory of exchange of heat. According to this theory, all bodies at all temperatures above 0K  radiate thermal energy and at the same time, they absorb radiation received from the surroundings. The amount of thermal radiation emitted per unit time depends on the nature of emitting surface, its area and its temperature.

Hotter bodies radiate at higher rate than the cooler bodies. For a body, the absorbed radiation increases the KE of the constituent atoms oscillating about their mean positions. The body itself also radiates, therefore its energy decreases, causing lowering of temperature. If a body radiates more than it absorbs, its temperature decreases and vice versa. When the rate of absorption of radiation is same as the rate of emission of radiation, the temperature of the body remains constant and the body is said to be in thermal equilibrium with its surroundings. At room temperature, the thermal radiation corresponds to wavelengths longer than those of visible light and hence we do not see them. 

Amount of heat radiated by a body depends on:

1. The absolute temperature of the body (T)

2. The nature of the body – the material, nature of surface – polished or not, etc.

3. Surface area of the body (A)

4. Time duration of for which body emits radiation (t).

The amount of heat radiated, Q, is directly proportional to the surface area (A) and time duration (t). Therefore, emissive power or radiant power, R, of the body, at a given temperature T is given as,

R= Q/At

Coefficient of emission or emissivity: 

The coefficient of emission or emissivity (e) of a given surface is the ratio of the emissive power R of the surface to the emissive power RB  of a perfect black surface, at the same temperature. 

e = R/RB

For a perfect blackbody e = 1, whereas for a perfect reflector e = 0. For an ordinary body, 0 < e < 1 depending on the nature of the surface, e.g., emissivity of copper is 0.3. Emissivity is larger for rough surfaces and smaller for smooth and polished surfaces. Emissivity also varies with temperature and wavelength of radiation to some extent.