Electromagnetic radiation is the primary source of astronomical information.

In particular, until the early 1930s astronomy was all based on the use of telescopes

that extended the power of the human eye, but were restricted to the

collection of visible light. In general, the sources of astronomical electromagnetic radiation and other sources of astronomical information are what we call visible matter. This chapter introduces some key concepts and notation that characterize light and the collection of light for astronomical purposes. It addresses the main types of information that we may extract from the observations, by means of imaging and spectroscopy, recalling the difference between apparent and intrinsic properties of the astronomical sources and the fact that the light from distant sources is often a mixture of photons from different stars or different

components. This serves as an excuse for a quick introduction to important

concepts, such as stellar populations, mass-to-light ratios, mean motions, and

velocity dispersions. In closing the chapter, a method is described to measure the distance to a stellar system based on the application of a very simple dynamical model to a suitable set of observations.