In procedures like endoscopy, colonoscopy, and keyhole surgery, doctors use thin, flexible tubes called fiber optic scopes to see inside the body without large incisions. Inside these scopes, fiber optic cables transmit light to illuminate internal organs.

Total internal reflection makes this possible. When light enters the fiber, it reflects off the inner walls without escaping, as long as it hits at an angle greater than the critical angle. This allows the light to travel down the length of the fiber, even if it bends, reaching deep areas inside the body. This reflected light also brings back images, letting doctors see and examine tissues in real time, which helps in diagnosing and treating conditions with minimal invasive procedures.