The sky overhead encourages us to wonder about the nature of the universe—places so distant that we will likely never visit them easily. Sunlight illuminates our day and twinkling stars decorate our nighttime sky. All the glowing objects in the sky reveal themselves to us, even though they are very far away, by sending their light our way. Because these objects are so distant, the only information we have about them comes from the light we can capture with our telescopes. Fortunately, the physical properties of light are predictable and well understood. In this chapter we will first consider the basic properties of light; then we will discover how astronomers use light to reveal temperature, chemical composition, and motion through space of all the glowing objects in the sky. We will conclude with a discussion of how astronomers capture and enhance starlight in modern telescopes, in order to study it. The technology developed for the most modern of telescopes influences other technologies, including the high-quality cameras in cell phones and the lightweight materials they are made of.