Thermal radiation (heat waves) — Which statements correctly describe the propagation and reflection behavior of thermal radiation?

Introduction:
Heat transfer by radiation involves electromagnetic waves in the infrared and adjacent spectra. Unlike conduction and convection, radiation does not require a material medium and obeys geometric optics approximations in many engineering contexts. Knowing its basic propagation and reflection properties underpins furnace design, solar collectors, and thermal shielding.

Given Data / Assumptions:

• Classical (non-quantum) description of thermal radiation for macroscopic engineering systems.
• Surfaces may be idealized as diffuse/gray or as polished reflectors for conceptual clarity.
• Vacuum may exist between radiating surfaces.

Concept / Approach:

Radiation is an electromagnetic phenomenon; therefore it propagates through a vacuum and can be reflected, absorbed, or transmitted depending on surface optical properties. In the far field or when apertures are small relative to wavelength, ray approximations treat radiation as traveling in straight lines. Polished metals (high reflectivity in the IR) can specularly reflect a significant portion of incident thermal radiation.

Step-by-Step Solution:

Verification / Alternative check:

Radiative exchange between parallel plates in vacuum is a standard textbook problem; reflectivity of polished aluminum/steel in the IR confirms (c).

Why Other Options Are Wrong:

A denies vacuum propagation; E is overly restrictive since both specular and diffuse reflection are possible depending on surface roughness and material.

Common Pitfalls:

Confusing thermal radiation with heat conduction, which does require matter; or assuming only visible light reflects off mirrors while IR does not (it often does, depending on material).

Final Answer:

Both (b) and (c).