Which principal is used by fiber optics where light is trapped inside a substance?

The principal used by fiber optics to trap light inside a substance is called total internal reflection. This phenomenon occurs when light traveling within a medium hits the boundary with a less dense medium at an angle greater than the critical angle, causing the light to reflect back into the denser medium rather than refracting out. This principle allows fiber optic cables to effectively transmit light signals over long distances with minimal loss.