Hot things glow short, cool things glow long. That’s the whole law in plain terms.

The Sun is super hot, so it glows in visible light (we can see it).
The Earth is cool, so it glows in thermal infrared (we can’t see it, but our sensors can).

That’s exactly why we use VIS/NIR sensors to capture reflected sunlight, and thermal infrared sensors to capture Earth’s own heat — at night, in the dark, all the time.

🔬 Science / formula

λ_max = 2897.8 / T (µm, T in K)

Sun (~6000 K): λ_max ≈ 0.483 µm (visible)
Earth (~300 K): λ_max ≈ 9.66 µm (thermal IR)

Hotter → shorter peak wavelength.

💡

Hot = short, cool = long. Sun hot → 0.48 µm (visible). Earth cool → 9.66 µm (thermal IR). That's why we use VIS/NIR for reflected solar and thermal IR for emitted Earth.

emr/wien-law