A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an instrument designed for the observation of remote objects. The term usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation and for other signal types.
An optical telescope gathers and focuses visible light and other electromagnetic radiation. Telescopes increase the apparent angular size of distant objects, as well as their apparent brightness. Telescopes work by employing one or more curved optical elements - lenses or mirrors - to gather light or other electromagnetic radiation and bring that light or radiation to a focus, where the image can be observed, photographed or studied. Optical telescopes are used for astronomy and in many non-astronomical instruments including theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses and spyglasses.
Single-dish Radio telescopes are focusing radio antennae often having a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. As of 2005, the current record array size is many times the width of the Earth, utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and Aperture Masking Interferometry at single telescopes.
X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola. High energy particle telescopes detect a flux of particles, usually originating at an astrono
for more details, you can visit the below link.
Btw, 上層的朋友答錯了, 因為他答了microscope顯微鏡