Ideal Black Body
Ideal Black Body :- For a body surface which absorbs all incident thermal radiations at low temperature irrespective of their wave length and emitted out all these absorbed radiations at high temperature assumed to be an ideal black body surface.
Characteristics of an Ideal Black Body
(1). Absorptivity and Reflectivity :
For an ideal black body, the absorptivity (a) is equal to 1, meaning it absorbs 100% of the incident radiation. Its reflectivity (r) and transmissivity (t) are zero, indicating no reflection or transmission of radiation. The emissivity (e) is also 1, which means it emits radiation with maximum efficiency.
(2). Dependence on Temperature :
The nature of the radiation emitted from the surface of an ideal black body depends solely on its temperature. The emitted radiation is often referred to as full radiation or white radiation because it contains all wavelengths.
(3). Spectral Energy Distribution :
At any given temperature, the spectral energy distribution curve of an ideal black body is continuous. This continuity implies that if the radiation spectrum of a heat source is continuous, it can be classified as similar to an ideal black body. Examples include sources like kerosene lamps, oil lamps, and heating filaments.
(4). Absorption and Emission Behavior :
At low temperatures, the surface of an ideal black body acts as a perfect absorber, while at high temperatures, it functions as an efficient emitter.
(5). Color of an Ideal Black Body :
Contrary to what the name suggests, an ideal black body does not necessarily have to be black in color. For example, the Sun behaves like an ideal black body despite its bright appearance.
(6). Experimental Model :
Ferry’s Ideal Black Body : Ferry’s ideal black body is a practical model designed to approximate the behavior of a perfect black body. It consists of a hollow enclosure with a small pinhole (or aperture) in its wall. Radiation entering the hole undergoes multiple reflections inside the cavity, and very little escapes, making it effectively absorb all incident radiation.
- The inner surface of the enclosure is coated with a highly absorptive material (like lamp black).
- The small hole acts as a perfect absorber since incoming radiation is unlikely to reflect back out.
- It serves as a good experimental approximation of an ideal black body, especially in laboratory settings.
