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 types of electromagnetic radiation. The optics of a telescope must be almost perfect, meaning 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.

There are two main types of telescopes: refracting telescopes and reflecting telescopes. Refracting telescopes were the first type of telescope, and their invention is attributed to a Dutch lensmaker, Hans Lippershey, in 1608. They have 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. If there are any flaws in a telescope’s optics (e.g., the mirrors or lenses), then the image created will appear warped or out-of-focus and blurry.

Telescopes may also be classified by location: ground telescope, space telescope, or flying telescope. They may also be classified by whether they are operated by professional astronomers or amateur astronomers. A vehicle or permanent campus containing one or more telescopes or other instruments is called an observatory.

In summary, a telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. It works by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other types of electromagnetic radiation. There are two main types of telescopes: refracting telescopes and reflecting telescopes. Telescopes may also be classified by location and whether they are operated by professional or amateur astronomers.