What describes the percentage of absorbed energy that a material can radiate away?

• A. Albedo
• B. Reflectivity
• C. Emissivity
• D. Conductivity

Answer: (C)

The percentage of absorbed energy that a material can radiate away is termed emissivity. Emissivity is a measure of a material's ability to emit infrared energy compared to a perfect black body, which has an emissivity of 1. This property is crucial in various applications, including roofing, as it affects how well a building can manage heat. High emissivity materials can transfer heat more effectively, which can be beneficial in reducing cooling costs for buildings.

Albedo refers to the reflectivity of a surface, particularly how much sunlight it reflects rather than absorbs. Reflectivity is the fraction of incident light or radiant energy that is reflected by a surface. Both albedo and reflectivity focus on how a material interacts with incoming radiation rather than its ability to emit absorbed energy. Conductivity, on the other hand, pertains to the rate at which heat or electricity can pass through a material, which is not directly related to how much energy a surface can radiate away. Understanding these definitions clarifies why emissivity is the correct term for describing the ability of a material to radiate absorbed energy.