Nearly everything transmitted across the internet travels, for the longest and fastest portion of its journey, as pulses of light inside a strand of glass thinner than a human hair. Copper conducted the world's communications for roughly a century, and over distance it has been almost entirely replaced by glass, not for reasons of fashion but because the underlying physics of guiding light is simply superior to the physics of pushing electrical signals down a wire.
The principle that makes an optical fiber work is total internal reflection. When light traveling through a denser medium strikes the boundary with a less dense one at a sufficiently shallow angle, it does not pass through, it reflects completely, losing almost no energy at the interface. A fiber is engineered around exactly this effect, a central core of glass surrounded by a cladding of glass with a slightly lower refractive index, so that light injected into the core at the right angle strikes the core cladding boundary beyond the critical angle and reflects, again and again, remaining trapped within the core even as the fiber bends and routes around obstacles. The signal completes its journey by riding thousands of these near perfect reflections without escaping the glass that guides it.
Electrical signals in copper attenuate with distance, pick up electromagnetic interference from their surroundings, and degrade further as the signal frequency rises, so a copper link over any real distance requires frequent regeneration and still cannot approach the capacity modern communication demands. Light confined in high purity glass loses so little energy that a signal can travel tens of kilometers before it needs amplification, it is immune to electromagnetic interference, and the extremely high frequency of light permits a data carrying capacity that copper cannot rival. Two engineering achievements make this practical beyond the physics itself, glass refined to a purity that lets light traverse kilometers with only slight dimming, and wavelength division multiplexing, the technique of sending many independent signals through a single fiber at once as different wavelengths of light that do not interfere with one another, so that one strand carries the traffic of many.
The result is that a cable no thicker than a garden hose can carry the communications of an entire region, its capacity limited less by the medium than by the electronics at each end. When a page loads without perceptible delay across a continent, the physical reality behind it is a beam of light traveling down a flawless glass thread, reflecting perfectly off a boundary it never crosses, and arriving so little changed that the distance scarcely registered. Fiber optics is not an improved wire, it is a fundamentally different way of moving information, and understanding that difference explains why the physical internet, for all its scale, ultimately rests on the precise and reliable behavior of light in glass.