Telescope
A telescope collects light or other radiation from distant objects and magnifies it so astronomers can study stars, planets, and galaxies.
Definition
A telescope is an instrument that makes faraway things look closer and brighter by gathering far more electromagnetic radiation, visible light included, than the eye ever could and bringing it to a focus, using lenses in a refracting telescope or mirrors in a reflecting one; nearly all large research telescopes are reflectors because big mirrors are easier to make and support than big lenses. Beyond visible light, telescopes exist for radio waves, infrared, ultraviolet, X-rays, and gamma rays, and some are placed in space specifically to avoid the blurring and absorption caused by Earth's atmosphere. Light-gathering power scales with the square of a telescope's aperture $D$, and its diffraction-limited angular resolution works out to about $\theta \approx 1.22\,\lambda/D$ radians for wavelength $\lambda$, though ground-based optical telescopes are limited by atmospheric turbulence to roughly $1$ arcsecond unless adaptive optics correct the wavefront in real time. Interferometry gets around that limit by combining separated telescopes to reach the resolution of one aperture as wide as their spacing, which is how the Event Horizon Telescope imaged a black hole's shadow, and spectrographs attached to telescopes disperse light to reveal composition, temperature, and velocity, since most of what astronomers know comes from spectra rather than images.
Example
Galileo pointed a small telescope at the sky in 1609 and discovered four moons circling Jupiter, and even ordinary binoculars will show those same moons today. The Hubble Space Telescope, launched in 1990, carries a $2.4\ \text{m}$ mirror and has orbited above the atmosphere for decades, while the James Webb Space Telescope, launched in December 2021, carries a $6.5\ \text{m}$ mirror and observes in infrared light to see the earliest galaxies. For JWST at $\lambda = 2\ \mu\text{m}$ and $D = 6.5\ \text{m}$, the resolution formula gives $\theta \approx 1.22 \times 2 \times 10^{-6} / 6.5 \approx 3.8 \times 10^{-7}$ radians, about $0.08$ arcseconds, sharp enough to separate two objects only a few hundred meters apart on the Moon.
Key Insight
A telescope's most important job is not magnifying but collecting light, since a wider one gathers more of it and can see fainter, more distant objects; doubling a mirror's diameter quadruples its light-gathering area, which is why JWST's $6.5\ \text{m}$ mirror collects over $7$ times more light than Hubble's $2.4\ \text{m}$ one. In a deeper sense every telescope works as a time machine and a chemistry lab combined: because light travels at a finite speed and carries the spectral fingerprints of the atoms that emitted it, a telescope reveals what a distant object was made of and doing when its light first left, sometimes billions of years ago.