Visible light — only the narrow band 0.4 – 0.7 µm is detectable by the human eye, out of a spectrum that spans more than twelve orders of magnitude. Nearly all of “remote sensing” lives outside what we can see.

• Violet ≈ 0.4 µm, blue ≈ 0.45, green ≈ 0.55, red ≈ 0.65–0.7 µm.