Radiative properties of media: which of the following are generally considered opaque to thermal radiation over typical engineering thicknesses?

Introduction:
Opacity describes whether a medium allows radiation to pass through. In heat transfer practice, most common solids and liquids are treated as opaque to thermal radiation, whereas many gases are partially transparent over engineering path lengths, with absorption in specific spectral bands.

Given Data / Assumptions:

• Thermal radiation (infrared) wavelengths of interest.
• Typical engineering thicknesses (millimeters to centimeters).
• Homogeneous, continuum media behavior.

Concept / Approach:
For radiation analysis, surfaces of solids and free surfaces of liquids generally act as opaque boundaries with emissivity/absorptivity defining surface exchange. Gases like CO2 and H2O vapor absorb selectively but are often semi-transparent; path length and concentration determine their effective opacity. Hence, as a broad classification, solids and liquids are “opaque,” gases are not necessarily so.

Step-by-Step Solution:
Classify phases by typical radiative behavior.Recognize solids and liquids are treated as opaque in most designs.Select “Both (b) and (c)” accordingly.

Verification / Alternative check:
Standard radiation texts model solids/liquids as gray, opaque surfaces, while participating media treatment is reserved for gases/flames and certain semitransparent solids (e.g., glass at specific wavelengths).

Why Other Options Are Wrong:

• Gases: Often semi-transparent; not broadly opaque.
• Individual (b) or (c): Each true, but the combined answer reflects both phases.

Common Pitfalls:
Overgeneralizing: some solids (e.g., thin glass) can be semitransparent in parts of the spectrum, but the engineering default for thermal analysis is opacity.

Final Answer:
Both (b) and (c)