Explain view factors in radiation heat transfer and the reciprocity rule.

View factor (F₁₂) is the fraction of radiation leaving surface 1 that directly reaches surface 2, depending only on geometry. Key rules: Summation rule (ΣF₁ⱼ = 1 for an enclosure), Reciprocity (A₁F₁₂ = A₂F₂₁), Self-view factor for concave surfaces (F₁₁ > 0). For parallel plates of equal size with separation H and dimension L: F₁₂ approaches 1 as H→0. Heat exchange between two surfaces: Q₁₂ = A₁F₁₂σ(T₁⁴-T₂⁴) for blackbodies. View factors are tabulated or calculated using integration. Understanding view factors is essential for furnace design, spacecraft thermal control, and industrial heating applications.