English: Waveguides can be described by ray optics and total internal reflection. a. Light passing into a medium with a higher refractive index bends toward the normal. b. Each ray in the low index medium (black) maps to a ray in the high index medium (blue). The remaining rays (red) in the high index medium are trapped by a process called total internal reflection. c. A slab of high index material will trap rays at both top and bottom surfaces, guiding the light and forming a waveguide. d. This is the principle behind an optical fiber.