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).