- Electromagnetic waves carry energy through space at the speed of light in a vacuum. When light travels from one medium to another, its speed and direction change due to the media's different refractive indices. This is called refraction and is described by Snell's law. - In an optical fiber, total internal reflection guides light through the higher index core by reflecting it at the core-cladding interface. For this to occur, the incidence angle must exceed the critical angle. - The numerical aperture specifies an optical fiber's light acceptance capabilities based on the refractive index difference between its core and cladding. It determines the maximum acceptance angle for light entering the fiber.