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, monocular, binoculars, camera lenses and spyglasses.

Compound Microscope:

A compound microscope is a high power (high magnification) microscope that uses a compound lens system. A compound microscope has multiple lenses: the objective lens (typically 4x, 10x, 40x or 100x) is compounded (multiplied) by the eyepiece lens (typically 10x) to obtain a high magnification of 40x, 100x, 400x and 1000x. Higher magnification is achieved by using two lenses rather than just a single magnifying lens. While the eyepieces and the objective lenses create high magnification, a condenser beneath the stage focuses the light directly into the sample.

Typically, a compound microscope is used for viewing samples at high magnification (40 - 1000x), which is achieved by the combined effect of two sets of lenses: the ocular lens (in the eyepiece) and the objective lenses (close to the sample).