Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
(such as visible light). The first known practical telescopes were invented in the Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
at the beginning of the 1600s (the 17th century), using glass lenses. They found use in terrestrial applications and astronomy.
Within a few decades, the reflecting telescope
A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
was invented, which used mirrors.