Question

For an absolutely white or specular body-
(where α = absorptivity, ρ = reflectivity, τ = transmissivity)
1. α = 1, ρ = 0 and τ = 0
2. ρ = 1 and α = τ = 0
3. τ = 1 and α = ρ = 0
4. α + τ = 1 and ρ = 0

Answer 1

Correct Answer - Option 2 : ρ = 1 and α = τ = 0

Concept:

White body

• A body that is called a white body is one that reflects almost all radiations incident up on it and does not absorb or transmit any part of it.
• For a white body, α = τ = 0, ρ = 1

Black body

• A black body is an object that absorbs all the radiant energy reaching its surface.
• No actual body is perfectly black; the concept of a black body is an idealization with which the radiation characteristics of real bodies can be compared.
• Properties of the black body:
  • It absorbs all the incident radiation falling on it and does not transmit or reflect regardless of wavelength and direction
  • It emits the maximum amount of thermal radiation at all wavelengths at any specified temperature
  • It is a diffuser emitter (i.e. the radiation emitted by a black body is independent of direction)
• Transparent body: τ = 1, α = ρ = 0
• Opaque body: τ = 0