A telescope is typically thought of as an optical instrument used to view distant objects by means of magnifying lenses. Telescopes have been around for over 400 years and have enabled professional and amateur astronomers alike to see far beyond the limits of the naked human eye. The most powerful telescopes of today are capable of detecting distant objects billions of light-years away from Earth, but this certainly wasn't always the case.
In regard to the telescope, there is a very common misconception that Galileo was this instrument's inventor. While Galileo was one of the first to turn a telescope skyward to observe celestial objects, he did not invent it. The existence of glass had been known since ancient Phoenicians discovered it while cooking on sand around 3500 B.C. As the centuries passed, lenses were developed to correct visual problems such as hyperopia and myopia, better known as farsightedness and nearsightedness. In fact, eyeglasses existed over 200 years before telescopes came into the picture.
As far as telescopes go themselves, Hans Lippershey, an optician from the Netherlands, is often credited as the inventor in 1608, when he found that holding two different lenses apart brought objects closer, but the objective here was to make a spying glass as opposed to checking out objects of interest in the solar system. In 1609, Galileo improved upon the design. While he was at it, he used this new instrument for a different purpose and discovered craters and mountains on the moon, Jupiter's Great Red Spot and four of the planet's largest moons, and the rings of Saturn.
A couple of years later, in 1611, Johannes Kepler suggested that telescopes could be made with convex lenses and eyepieces. For the next few decades, these types of refracting telescopes were widely used by astronomers of the day, but in the meantime, reflecting telescopes were also slowly being developed. As opposed to refracting telescopes that magnified objects with convex lenses, the reflecting type employed the use of concave mirrors to achieve the same results. For the next 300 years or so, both reflecting and refracting telescopes based on land continued to grow in size and thus reveal more of the cosmos. Then, in 1937, something happened.
The first radio telescope was constructed by a man named Grote Reber. This was a brand new type of telescope that detected radio waves from outer space. Radio astronomy was born and really took off following the end of the Second World War. Just as with land-based telescopes used for visual observation, radio telescopes evolved as well. In 1991, the first gamma-ray telescope, known as the Compton Gamma Ray Observatory, was built. During the latter part of the 20th century, X-ray, ultraviolet, and infrared telescopes were also developed. Still, the technology grew. Many readers may not know this, but as far back as 1962, the idea of a large, space-based telescope was already on the drawing board, but it would be another 15 years before construction on the famous Hubble Telescope would begin. This advanced instrument was finally finished by 1985, but wouldn't be deployed until 1990 due to the 1986 Challenger disaster, in which 7 American astronauts perished after the space shuttle exploded shortly after launch.
Despite this tragic setback, orbiting telescopes such as the Hubble, Chandra X-Ray Observatory, Keppler, Herschel and Spitzer have revealed more about the cosmos than anyone could have imagined even a few decades ago. Not only have planets outside this solar system been found, but some of the earliest events following the Big Bang have also now been observed. Since light travels at a finite speed, astronomers are seeing into the past. The further an object is from Earth, the longer time has passed. For example, if a person looking through a telescope happens to have their eyes fixed on a star that is 1,000 light-years away, it is being seen as it was in 1013 as opposed to 2013. Most physicists now believe the Big Bang signaled the beginning of the universe around 15 billion years ago, so it's quite interesting to realize that, as telescopes become even more advanced, future generations may be able to view the beginning of time itself.