A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Optical telescopes are telescopes that gather and focus visible light from distant objects and make the objects appear bigger and closer. The earliest true optical telescopes, which were made in Europe in the late 16th century, were optical telescopes. They used a combination of two curved lenses to focus light. The term telescope was coined by the Italian scientist and mathematician Galileo Galilei (1564–1642). Galileo built his first telescope in 1608 and... Show more

A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.
Optical telescopes are telescopes that gather and focus visible light from distant objects and make the objects appear bigger and closer. The earliest true optical telescopes, which were made in Europe in the late 16th century, were optical telescopes. They used a combination of two curved lenses to focus light. The term telescope was coined by the Italian scientist and mathematician Galileo Galilei (1564–1642). Galileo built his first telescope in 1608 and subsequently made many improvements to telescope design, which allowed him to make important observations of our solar system and galaxy.    
Optical telescopes that use lenses focus light by refracting, or bending, it. These optical telescopes are called refracting telescopes. The earliest telescopes, including Galileo’s, were all refracting telescopes. Many of the small telescopes used by amateur astronomers today are refracting telescopes. The largest refracting telescope in the world is at the University of Chicago’s Yerkes Observatory in Wisconsin and was built in 1897. Its largest lens has a diameter of 102 cm.    
Around 1670, another famous scientist and mathematician—Sir Isaac Newton (1643–1727)—built a new kind of optical telescope using mirrors instead of lenses. The mirrors were curved to reflect and focus light. This type of telescope is called a reflecting telescope. Reflecting telescopes have advantages over refracting telescopes. One  advantage is that mirrors are easier than lenses to make precisely. The mirrors in a reflecting telescope are also much lighter than the heavy glass lenses in a refracting telescope, so reflecting telescopes can be larger without becoming too heavy. Larger telescopes can collect more light, so they allow scientists to view dimmer or more distant objects.    
The largest optical telescopes in the world today are reflecting telescopes.  

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