Most of what we know about the Universe comes from information that has been carried to us by light. Thus, it is appropriate to begin our study of the Universe by examining the properties of light and the instruments that are used to detect it.
In modern physics, light or electromagnetic radiation may be viewed in one of two complementary ways: as a wave in an abstract electromagnetic field, or as a stream of massless particles called photons. Although either is an acceptable description of light, for our purposes in introductory astronomy the wave description will be more useful.