1. The document discusses optical fibers, which are thin strands of glass that transmit light through the phenomenon of total internal reflection. 2. Optical fibers consist of a core surrounded by cladding material with a lower refractive index. Light is guided through the core due to total internal reflection at the core-cladding boundary. 3. The document discusses different types of optical fibers including single mode fibers, which transmit only one wavelength/mode of light, and multimode fibers, which transmit multiple modes of light due to their larger core diameter.