A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally, it was an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects – an optical telescope. Nowadays, the word "telescope" is defined as a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. Telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point.

There are two main types of telescopes: refracting and reflecting telescopes. Refracting telescopes were the first type of telescope and use a curved lens at one end that focuses the light down a long tube towards a second lens, called the eyepiece, which magnifies the image. Reflecting telescopes, on the other hand, use mirrors to collect and focus the light towards the eyepiece. Mirrors are lighter than lenses and they are also easier to shape into a smooth and perfect surface.

The optics of a telescope must be almost perfect. That means the mirrors and lenses have to be just the right shape to concentrate the light. They can’t have any spots, scratches, or other flaws. If they do have such problems, the image gets warped or blurry and is difficult to see.

Telescopes are used for astronomy and in many non-astronomical instruments, including theodolites (including transits), spotting scopes, monoculars, binoculars, camera lenses, and spyglasses. Space telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope have allowed us to capture views of galaxies and nebulas far away from our own solar system.

In summary, a telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. There are two main types of telescopes: refracting and reflecting telescopes. The optics of a telescope must be almost perfect to produce a clear image.