A body whose absorptivity does not vary with temperature and wavelength of the incident ray is known as
gray body
Understanding Absorptivity and Thermal Radiation
When thermal radiation falls on a body, some of it is absorbed, some is reflected, and some may be transmitted through the body. The fraction of the incident radiation that is absorbed by the body is called its absorptivity, often denoted by $\alpha$.
The absorptivity of a real body can depend on several factors, including:
- The temperature of the body.
- The temperature of the source of radiation.
- The wavelength of the incident radiation.
- The direction of the incident radiation.
Characteristics of Different Bodies
Let's look at how different types of idealized bodies interact with incident thermal radiation based on their absorptivity:
- Black Body: An ideal black body is defined as a body that absorbs all incident thermal radiation, regardless of its wavelength, direction, or the temperature of the source. Its absorptivity ($\alpha$) is equal to 1. It also emits the maximum possible thermal radiation for a given temperature according to Planck's law.
- White Body: A white body is defined as a body that reflects all incident thermal radiation. Its reflectivity is 1, and consequently, its absorptivity ($\alpha$) is 0.
- Opaque Body: An opaque body is one through which no thermal radiation can be transmitted. For an opaque body, the incident radiation is either absorbed or reflected. The transmissivity is 0.
- Gray Body: A gray body is an idealized body that has a constant absorptivity ($\alpha$) that is less than 1 but greater than 0. Crucially, this constant absorptivity does not vary with the temperature of the body or the wavelength of the incident radiation.
Gray Body Explained
The question describes a body whose absorptivity does not change with temperature and wavelength of the incident ray. This specific characteristic matches the definition of a gray body.
A gray body is often used as a simplification in thermal radiation calculations for real surfaces, which usually have absorptivity that varies with wavelength and temperature. By assuming a constant average absorptivity, calculations become much simpler while still providing a reasonable approximation for many engineering applications.
| Body Type | Absorptivity ($\alpha$) | Dependency on Temperature & Wavelength |
|---|---|---|
| Black Body | 1 | Constant (by definition) |
| White Body | 0 | Constant (by definition) |
| Gray Body | Constant (0 < $\alpha$ < 1) | Does not vary |
| Opaque Body | Variable (0 < $\alpha$ < 1) | Can vary (depending on the material properties) |
Based on the definitions, a body whose absorptivity remains constant irrespective of temperature and the wavelength of the incident radiation is known as a gray body.
Important Questions from Radiation
The heat of the sun reaches us according to
Heat is transferred from an electric bulb by ______.
A wave of radiation falls on a body, 35% of the radiation is reflected back. If transmissivity of the body is 0.25, then emissivity is:
A room window (consisting of a vertical sheet of plane glass) is exposed to direct sunshine at a strength of 1000 W/m2. The window is pointing due south, while the sun is in the southwest, 30° above the horizon. Estimate the amount of solar energy in W/m2 reflected by the window. Assume glass to be gray with ρ(reflectivity) = 0.08.
- Radiosity is defined as _______.
