In this chapter, the reader will be acquainted with the meaning of “light” and the main terms that recur frequently in deliberations on lighting (Schanda and Madár 2007, ompson 2011).
2.1 What Is Light? Light is a form of electromagnetic energy (Vandergri 2008). Although light travels very swiftly, its speed is not unlimited but reckonable. In vacuum, light propagates at a steady speed c = 2.99 × 108 m/s, which is taken as a universal constant. In nonvacuum media, the speed changes, for example, in air, light travels at 0.03% lower speed, while in glass, it moves 30.0% slower than in vacuum.