When sunlight passes through a prism, we see it split into a rainbow of colors. This effect can be explained by understanding light as an electromagnetic wave. (We therefore sometimes refer to light as electromagnetic radiation.) As the light passes through the prism, shorter wavelengths are bent more than longer ones, which produces the familiar rainbow. Longer wavelengths (red) have lower energy than shorter wavelengths (blue). We call a graph of how much light an object emits at every wavelength a spectrum.
Our eyes are fine-tuned to see only the tiny range of wavelengths that are most strongly emitted by our Sun. (The amount of light emitted at each wavelength is largely determined by the physics of blackbody radiation.) However, there are many kinds of light that we can’t see, spanning a wide range of wavelengths. A diagram of the full electromagmetic spectrum is shown below. The lowest energy light has wavelengths as big as mountains, while the highest energy light has wavelengths as small as the width of atoms. Astronomical processes produce light at all of these different wavelengths. By looking at the Universe with telescopes that can see different portions of the electromagnetic spectrum, we can see different processes occurring in the same region of the sky and gain new information about what is happening there. For example, hot young stars produce a lot of light in the ultraviolet and X-ray portions of the electromagnetic spectrum while older cooler stars do not. Additionally, while dust in the ISM can only be seen in visible light when light from stars reflects off it or backlights it, it glows quite brightly at infrared and radio wavelengths.