In radiative heat transfer, a gray surface is one _____
Gray Surface Definition in Radiative Heat Transfer
A gray surface in the context of radiative heat transfer is defined by its thermal emission properties.
Understanding Gray Surfaces
The defining characteristic of a gray surface relates to how its emissivity behaves across different wavelengths of radiation.
- Definition: A gray surface is one whose emissivity (ε) is constant and independent of the wavelength (λ) of the emitted radiation.
- Implication: This means the ratio of its emissive power to that of a blackbody at the same temperature is the same for all wavelengths.
Analyzing Incorrect Options
Let's look at why the other options are not the correct definition:
- Option 1 (Visual Appearance): A surface appearing gray visually relates to its visible light reflectivity, not its thermal radiative properties across all wavelengths. A surface can look gray but have wavelength-dependent emissivity.
- Option 3 (Zero Reflectivity): A surface with zero reflectivity (ρ = 0) is either a blackbody (if absorptivity α = 1) or a perfectly transparent surface. A gray surface does not necessarily have zero reflectivity; its properties are uniform across wavelengths, but not necessarily zero.
- Option 4 (Equal Brightness): Appearing equally bright from all directions describes a diffuse surface, which is related to the directional properties of emission or reflection, not the spectral (wavelength) properties.
Therefore, the correct definition hinges on the spectral independence of emissivity.
Important Questions from Radiation
Stefan Boltzmann's constant is expressed in the unit-
A body whose absorptivity does not vary with temperature and wavelength of the incident ray is known as
The process of heat transfer from a hot body to a cold body in a straight line, without affecting the intervening medium, is known as ______.
Heat is transferred from an electric bulb by ______.
- Radiosity is defined as _______.
