Question

What is a blackbody? How do real bodies differ from blackbodies?

Short answer

Answer: Real bodies differ from blackbodies in terms of their emissivity and absorption because they do not perfectly emit or absorb radiation like a blackbody. A real body has an emissivity value between 0 and 1, meaning they emit less thermal radiation than a blackbody at the same temperature. In terms of absorption, real bodies have varying absorption coefficients and may reflect and transmit a portion of incident radiation, unlike a blackbody which perfectly absorbs all incident radiation with an absorption coefficient of 1.

Step-by-step solution

Defining a Blackbody

A blackbody is an idealized object that absorbs all incident electromagnetic radiation, regardless of wavelength or angle. As a result of this absorption, it also emits radiation according to its temperature, which is described by the blackbody radiation law. It is important to note that no perfect blackbodies exist in reality, but certain materials can approximate the properties of a blackbody well under certain circumstances.

Characteristics of a Blackbody

A blackbody has two main characteristics: 1. It absorbs all incident radiation, with an absorption coefficient of 1. This means that when a photon interacts with the blackbody, there is no reflection or transmission; the photon is entirely absorbed. 2. The emitted radiation from a blackbody only depends on its temperature. The emission spectra for blackbodies at different temperatures can be found using Planck's law, which gives the spectral radiance as a function of wavelength and temperature.

Real Bodies vs. Blackbodies - Emissivity and Absorption

Real bodies differ from ideal blackbodies in several ways. Primarily, they exhibit varying levels of emissivity and absorption. Emissivity refers to the ability of a surface to emit thermal radiation as compared to the ideal blackbody at the same temperature, while absorption refers to the ability of a material to absorb incident radiation compared to the ideal blackbody. A real body has an emissivity value (denoted by epsilon, \(\epsilon\)) that ranges between 0 and 1, where a perfect blackbody has an emissivity of 1. It means that the emitted thermal radiation from a real body is always less than that of a blackbody at the same temperature. In addition, the absorption coefficients of real bodies also vary. While an ideal blackbody's absorption coefficient is 1, real bodies can have different coefficients depending on the wavelength of incident radiation and the properties of the material. Real bodies reflect and transmit a portion of the incident radiation instead of absorbing it completely. In summary, real bodies differ from blackbodies primarily in terms of their emissivity and absorption, as their ability to emit and absorb radiation is not perfect like that of a blackbody.