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Updated: Jun 21, 2021

What is a Telescope?

A Telescope is an optical instrument that is designed to make distant object appear closer. Telescopes are used by astronomers, astrophysicists, and many amateur star gazers. Optical telescopes come in all shapes and sizes but they all work in a similar way.

there are three different types of telescopes Optical Telescopes, Radio telescopes, and Space telescopes.

Fun fact the smallest space telescope's width and length is only 20 Centimeter and weighs less than 7 kilogram; The worlds largest optical telescope the Gran Telescopio Canarias has an aperture of 10.4 meter.

Optical Telescopes

Optical telescopes work by using either lenses or curved mirrors or combination of lenses and mirrors.


A Telescope made with lenses is called a Refracting Telescope.

A Refracting Telescope as the name suggests works on the Law of Refraction.

The Objective lens is a Convex lens and the Ocular lens is a Concave lens.

Light travelling from a distant object comes as parallel rays; when these rays enter the objective lens they refract and because they passed through a convex lens; the light rays are bent towards each other inside the tube where they converge until they meet at a focal point; (because the light rays are crossing over each other the image will be flipped upside down. That won't be a problem because space doesn't really have an up or a down). The light rays diverge out into the Ocular lens where they straighten out; creating a virtual image on your retina.

Because the virtual image is closer to your eye than the real object; it has the effect of magnifying the image.

Credit: NASA/JPL-Caltech

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