What is the difference between a optical telescope and radio telescope?
Different types of telescopes look at different spectrums of light.
Different colors of light have differing wavelengths, and the collection of all of the colors of light is called the electromagnetic spectrum. The visible spectrum of light includes all of the colors that can be detected by the human eye, and it only makes up a very small portion of the total electromagnetic spectrum, about 400nm to 700nm.
Optical telescopes use polished mirrors or glass lenses to focus visible light as it comes in through the aperture. There are two main types of optical telescopes, reflectors and refractors. Reflecting telescopes tend to be much larger and use parabolic mirrors to focus light. Refracting telescopes use lenses to bend light as it passes throught them. These are limited by the size of the lenses, and therefore tend to be much smaller than reflectors.
Radio telescopes are used to study much longer wavelengths than visible light. Often, radio telescopes use a dish to focus the radio waves onto the receiver. The receiver then sends the signal to be recorded for astronomers to study. Some radio observatories, like the Very Large Array pictured below, use several radio telescopes at once to improve their signal.