The document discusses various concepts related to optical communication including: 1. The expressions for refractive index in graded index fibers and numerical aperture of step index fibers. 2. Definitions of mode-field diameter, linearly polarized waves, Snell's law, and the necessity of cladding for optical fibers. 3. Uses of optical fibers including transmitting information, optical imaging, and acting as light sources and sensors. 4. The principle of total internal reflection used for light guidance in optical fibers and definitions of step index and graded index fibers.