A photon has only energy and no mass; hence a beam of light consists of a flow of photons but no mass. The intensity of the beam is then directly proportional to the flow of photons; thus a higher-intensity beam has a greater flow of photons. Understanding Light Describing light as a flow of photons makes it relatively easy to visu- alize its absorption. In fact, this is what Albert Einstein did in 1905, to describe what is now referred to as the photoelectric effect. Dr. Einstein focused light onto a metal surface that was placed in a vacuum. At the same time he placed an electron detector above the metal to determine whether photons transferred their energy to electrons as the photons were absorbed by the metal. Dr. Einstein's photoelectric effect is illustrat- ed in Figure 2.1 and represents the basis for demonstrating the particle flow of light. Light as an Electromagnetic Wave Alternatively, light can be described as a wave. In the wonderful world of communications, it makes more sense to describe data transmission in terms of waves rather than individual particles, because nobody talks about the transmission of information over a wire circuit as a flow of electrons. Instead, we discuss how electromagnetic waves are modulated to convey information. As we proceed through this book, we can describe and discuss the flow of light through a fiber conductor with- out having to refer to the particle nature of light. Thus, in the remainder of this book we will describe and discuss light primarily in terms of an electromagnetic wave. An electromagnetic wave can be considered as a continuum of oscillat- ing electric and magnetic fields moving in a straight line at a constant velocity. For light, that velocity is approximately 186,000 miles per second (mi/s), or in the metric [Système International (SI)] system of measure- electron Figure 2.1 detector photon The photoelectric electron effect illustrates the transfer of light ener- gy to an electron and demonstrates the particle nature of light. metal in a vacuum Chapte