Snell's law describes how light refracts when passing from one medium to another, with the angle of refraction increasing as the angle of incidence increases. There is a critical angle where the angle of refraction is 90 degrees, above which total internal reflection occurs and all light is reflected rather than transmitted through the boundary. Refraction and total internal reflection are exploited in optical fibers to transmit light signals along its length, but dispersion can cause signals to spread out over time, potentially corrupting data transmission.