Used to discover moons, nebulae, and galaxies. This was the focus until around WW2. Optical telescopes Optical telescopes operate in the visible region of the electromagnetic spectrum. Visible radiation is that which we can see with our eyes. This form of radiation penetrates the Earth's atmosphere easily.
Optical telescopes operate on two fundamental properties of light, reflection and refraction.
Reflection is the phenomena associated with mirrors. Where light does not penetrate the mirror's surface, but rather is redirected by it. Refraction is the bending of light as it moves from one material to another. For example, light traveling between air and water is refracted. This is why you would have a hard time trying to pick up a coin on the bottom of a pool, while observing it from outside the water.