An instrument that collects and focuses electromagnetic radiation to observe distant objects.A telescope is an instrument that collects and focuses electromagnetic radiation to observe distant objects. While the word often evokes optical instruments, modern telescopes operate across the entire electromagnetic spectrum — from radio waves to gamma rays — and even detect gravitational waves and particles. The key function is always the same: gather more radiation than the naked eye can, concentrate it, and form an image or a spectrum.
The first telescopes were built in the early 17th century, with Hans Lippershey applying for a patent in 1608 and Galileo Galilei making his own in 1609. Isaac Newton built the first practical reflecting telescope in 1668.
Optical telescopes use lenses or mirrors to collect light and bring it to a focus. Refracting telescopes use lenses; reflecting telescopes use curved mirrors, which avoid chromatic aberration and can be made much larger. For other wavelengths, the design changes: radio telescopes use large dish antennas, X-ray telescopes use grazing-incidence mirrors, and gamma-ray telescopes rely on particle detectors. The larger the collecting area, the fainter the objects that can be seen, and the sharper the resolution.
Telescopes are used in astronomy to study planets, stars, galaxies, and cosmic phenomena, as well as in Earth observation, satellite tracking, and even in some laser communication systems.
The largest optical telescope mirrors are over 8 meters in diameter, but the largest radio telescope, FAST in China, spans 500 meters — bigger than 30 football fields.






