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**Wien's displacement law.** Gives the **peak-emission wavelength** of a blackbody: `λ_max = 2897.8 / T` (λ_max in µm, T in K) - **Sun (T ≈ 6000 K):** λ_max ≈ **0.483 µm** (green-blue visible light). - **Earth (T ≈ 300 K):** λ_max ≈ **9.66 µm** (thermal infrared). - **Consequence for sensors:** use visible/NIR to measure **reflected solar**, and thermal infrared (8–14 µm) to measure **emitted Earth** radiation. - **Hotter object → shorter peak wavelength** (why a stove burner glows red, then orange, then white).

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