Ferry's black body and what is perfectly black body?

What is perfectly black body?

A perfectly black body, also known as an ideal black body or a blackbody radiator, is an idealized physical concept in physics and thermodynamics. It refers to an object that absorbs all incoming electromagnetic radiation, regardless of wavelength or frequency, without reflecting or transmitting any of it. In other words, it absorbs all incident light and does not reflect or emit any light of its own.

The concept of a black body is important in understanding the behavior of thermal radiation and the principles of thermodynamics. According to Planck's law and the Stefan-Boltzmann law, a black body is a theoretical object that emits radiation with maximum efficiency at all wavelengths. It absorbs all radiation that falls upon it and re-emits it in a characteristic manner, known as black body radiation.

Black body radiation follows a specific distribution of wavelengths, known as the Planck distribution or Planck's law. The intensity and distribution of the emitted radiation depend solely on the temperature of the black body. At any given temperature, a black body emits radiation across a continuous spectrum of wavelengths, with higher temperatures leading to a higher intensity of radiation and a shift toward shorter wavelengths.

While a perfectly black body is an idealized concept and does not exist in reality, certain materials or structures can approximate the behavior of a black body to varying degrees across specific wavelength ranges. These materials are often used in applications such as thermal radiation detection, solar energy absorption, or in the creation of darkrooms and optical equipment.

Explain Ferry's black body

A perfectly black body is one which absorbs all the heat radiations incident on it. When such a body is placed inside an isothermal enclosure, it will emit the full radiation of the enclosure after it is in equilibrium with the enclosure. These radiations are independent of the nature of the substance. Such heat radiations in a uniform temperature enclosure are known as black body radiations.

A hollow copper sphere is taken and is coated with lamp black on its inner surface (Figure. 1). A

fine hole is made and a pointed projection is made just in front of the hole. When the radiations enter

the hole, they suffer multiple reflections and are completely absorbed. This body acts as a black body absorber. 

When this body is placed in a bath at a fixed temperature, the heat radiations come out of the

hole (Figure. 2). The hole acts as a black body radiator. It should be remembered that only the hole

and not the walls of the body, acts as the black body radiator.

Wien also constructed a black body in the form of a cylinder. This black body is commonly used currently. 

 

It consists of a hollow metallic cylinder and fitted with heating coils wound around it. The inner surface of the cylinder is coated with lamp black. The cylinder is placed in concentric porcelain tubes (Figure. 3). 

The temperature is measured with the help of a thermo couple arrangement. Heat radiations emerge out of the hole.The radiations from the inner chamber can be limited with the help of diaphragms provided on the inner side. This hole will act as a black body radiator.

Book Referred  : Atomic and Nuclear Physics - N. Subrahmanyam | Brij Lal | Jivan Seshan

                             S. Chand Publishing, 2008 - Science - 267 pages