Explain the terms absorptivity, reflectivity and transmissivity of radiant 10 energy. How are they related to each other for a black body and an opaque body?
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Absorptivity, reflectivity, and transmissivity are terms used to describe the behavior of radiant energy when it interacts with a material. These terms are related to each other and can be defined as follows:
1. Absorptivity: Absorptivity, also known as absorptance, is the measure of how much radiant energy is absorbed by a material when it strikes its surface. It is represented by the symbol "α." A material with high absorptivity absorbs a significant portion of the incident radiant energy, while a material with low absorptivity reflects or transmits most of the energy.
2. Reflectivity: Reflectivity, also known as reflectance or reflection coefficient, is the measure of how much radiant energy is reflected by a material when it strikes its surface. It is represented by the symbol "ρ." A material with high reflectivity reflects a large portion of the incident radiant energy, while a material with low reflectivity absorbs or transmits most of the energy.
3. Transmissivity: Transmissivity, also known as transmittance or transmission coefficient, is the measure of how much radiant energy passes through a material without being absorbed or reflected. It is represented by the symbol "τ." A material with high transmissivity allows a significant portion of the incident radiant energy to pass through, while a material with low transmissivity absorbs or reflects most of the energy.
For a black body, which is a theoretical concept that absorbs all incident radiation, the absorptivity is equal to 1 (α = 1), the reflectivity is zero (ρ = 0), and the transmissivity is also zero (τ = 0). This means that a black body absorbs all the radiant energy that strikes its surface without reflecting or transmitting any of it.
On the other hand, for an opaque body, which is a material that does not transmit any radiant energy, the absorptivity can vary from 0 to 1 (0 ≤ α ≤ 1), the reflectivity can also vary from 0 to 1 (0 ≤ ρ ≤ 1), and the transmissivity is zero (τ = 0). The sum of absorptivity, reflectivity, and transmissivity is always equal to 1 (α + ρ + τ = 1). This means that an opaque body can absorb, reflect, or a combination of both, but it does not transmit any radiant energy.
In summary, absorptivity, reflectivity, and transmissivity of radiant energy describe the behavior of how materials interact with incident radiation. For a black body, absorptivity is maximum while reflectivity and transmissivity are minimum. For an opaque body, absorptivity, reflectivity, and transmissivity can vary, but the sum of all three is always equal to 1.