The most important modern application of total internal reflection is the transmission of light through optical fibers. FIGURE 34.24a shows a laser beam shining into the end of a long, narrow-diameter glass tube. The light rays pass easily from the air into the glass, but they then impinge on the inside wall of the glass tube at an angle of incidence ${\theta }_1$ approaching $90\text{°}$. This is well above the critical angle, so the laser beam undergoes TIR and remains inside the glass. The laser beam continues to “bounce” its way down the tube as if the light were inside a pipe. Indeed, optical fibers are sometimes called “light pipes.” The rays are below the critical angle $({\theta }_1\approx 0)$ when they finally reach the end of the fiber, thus they refract out without difficulty and can be detected.

FIGURE 34.24 Light rays are confined within an optical fiber by total internal reflection.

FIGURE 34.24 Full Alternative Text

While a simple glass tube can transmit light, a glass-air boundary is not sufficiently reliable for commercial use. Any small scratch on the side of the tube alters the rays’ angle of incidence and allows leakage of light. FIGURE 34.24b shows the construction of a practical optical fiber. A small-diameter glass core is surrounded by a layer of glass cladding. The glasses used for the core and the cladding have $n_{\text{core}}>n_{\text{cladding}};$ thus light undergoes TIR at the core-cladding boundary and remains confined within the core. This boundary is not exposed to the environment and hence retains its integrity even under adverse conditions.

Even glass of the highest purity is not perfectly transparent. Absorption in the glass, even if very small, causes a gradual decrease in light intensity. The glass used for the core of optical fibers has a minimum absorption at a wavelength of $1.55\mu \text{m,}$ in the infrared, so this is the laser wavelength used for long-distance signal transmission. Light at this wavelength can travel hundreds of kilometers through a fiber without significant loss.