• $summary_{type}$: concept
• $word_{count}$: 150

The Earth's atmosphere is selectively transparent to EM radiation. Two main "windows": the optical window (visible + some near-IR, ~300 nm - 1 um) and the radio window (~1 cm - 10 m). Most UV is absorbed by ozone. Most IR is absorbed by water vapor and CO2 (greenhouse effect). X-rays and gamma rays are absorbed by the upper atmosphere. This is why optical and radio telescopes work from the ground, but UV, X-ray, and gamma-ray astronomy require space-based observatories (Hubble, Chandra, Fermi). Practical implications: (1) Satellite communication uses microwaves (penetrate ionosphere). (2) AM radio uses ionospheric reflection for long range. (3) FM/TV use line-of-sight (higher frequency, no ionospheric reflection). (4) Radar uses microwaves (good atmospheric transmission, good reflection from objects). (5) Fiber optics use near-IR (1550 nm — minimum absorption in glass).