ANS:B - black & rough

A black and rough surface refers to a surface that appears black in color and has a rough texture. When discussing heat transfer by radiation, such a surface is considered to have high emissivity and is often an efficient emitter and absorber of thermal radiation. Here's an explanation of why a black and rough surface is effective for heat transfer by radiation:

1. Black Surface:
   • A black surface is one that absorbs most of the incident radiation across a wide range of wavelengths. It appears black because it absorbs visible light rather than reflecting it.
   • Black surfaces are often referred to as "blackbodies," which are theoretical objects that absorb all incident radiation and emit radiation according to Planck's law. While true blackbodies are idealized, real surfaces that are black in appearance exhibit high emissivity across a range of wavelengths.
2. Rough Surface:
   • A rough surface has irregularities, bumps, or textures on its surface that cause incoming radiation to scatter rather than reflect uniformly.
   • Surface roughness can enhance the absorption of radiation by increasing the surface area available for absorption. Incoming radiation can penetrate into the surface irregularities, where it is absorbed and converted into thermal energy.
   • Additionally, the multiple reflections and scatterings within the rough surface contribute to its effective absorption and emission of radiation.
3. Combined Effect:
   • When a surface is both black and rough, it exhibits high emissivity across a wide range of wavelengths due to its ability to absorb and emit radiation efficiently.
   • The black color ensures that the surface absorbs most of the incident radiation, while the rough texture enhances absorption by increasing the surface area and promoting multiple reflections and scatterings within the surface.
   • As a result, a black and rough surface is highly effective for heat transfer by radiation because it efficiently absorbs thermal radiation from its surroundings and emits thermal radiation when heated.

In summary, a black and rough surface is effective for heat transfer by radiation because it combines the high emissivity of a black surface with the enhanced absorption properties of a rough surface. This combination allows the surface to absorb and emit thermal radiation efficiently, facilitating rapid heat transfer processes.