Rayleigh roughness criterion: for a surface to be considered optically rough to incident radiation at wavelength λ and incidence angle θ (from the normal), the root-mean-square height h must satisfy which inequality?

Introduction / Context:
Surface roughness relative to wavelength controls scattering behavior. In optical and microwave remote sensing, the Rayleigh criterion provides a simple threshold to distinguish between 'smooth' and 'rough' surfaces in terms of phase variations across the surface irregularities.

Given Data / Assumptions:

• λ is the sensing wavelength.
• θ is the incidence angle measured from the surface normal.
• h represents a characteristic height (e.g., RMS roughness) of surface irregularities.

Concept / Approach:
If height variations produce phase differences on the order of π/2 or greater across the illuminated facets, coherent specular reflection breaks down and the surface appears rough. Rayleigh's criterion expresses this as a threshold where h exceeds a fraction of the effective wavelength projected on the surface, commonly given by h > λ/(8 cos θ) for rough classification; for h below this threshold, the surface may be treated as optically smooth for that λ and θ.

Step-by-Step Solution:
Recall the Rayleigh roughness condition: h_threshold = λ / (8 cos θ).If h > h_threshold, the surface is rough; if h < h_threshold, it is smooth (for the given λ, θ).Identify the option that matches the 'greater than' condition for roughness.Select: h > λ/8 cos θ.

Verification / Alternative check:
Radar remote sensing texts apply this criterion to predict backscatter regimes (specular vs. diffuse) as a function of wavelength and grazing angle.

Why Other Options Are Wrong:

• Equality or 'less than' does not represent the roughness threshold.
• Expressions with undefined Δ are not standard.
• λ/4 cos θ sets a different, stricter threshold than Rayleigh's commonly used 1/8 factor.

Common Pitfalls:
Forgetting the cosine dependence on incidence angle; applying the criterion with wavelength units inconsistent with h.

Final Answer:
h > λ/8 cos θ