Almost every bit you have ever read on the internet — every email, every video frame, every bank transaction, every git push — has at some point traveled through a glass thread thinner than a human hair. The thread is silica, the carrier is light, and the engineering that gets data through it at terabit speeds across oceans is one of the most quietly extraordinary feats of the modern world.
This is the chapter where Chapter 0 (photons, lasers, and band gaps), Chapter 9 (Maxwell's equations, polarization, and the critical angle for total internal reflection), and Chapter 12 (digital modulation, error correction, and channel capacity) finally meet on the same page. Optical communications is electromagnetic propagation at hundreds of terahertz, modulated by digital symbols, guided by total internal reflection, detected by reverse-biased diodes, and amplified by the photons of erbium ions sitting in glass. Every concept from those chapters survives the trip from radio frequencies to optical frequencies. We just have to understand how each of them changes when the wavelength shrinks from meters to microns.
Take this chapter slowly. The intuition is more important than any single formula. Once you can answer the question "why does light go straighter through glass than electrons go through copper?" the rest follows.