In an optical fibre the signal is transmitted

Total internal reflection is the basic concept behind the optical fiber. Optical fiber employs the transmission of light down fibers of plastic or glass. Because the fibers are thin, light entering one is likely to strike the inside surface at an angle greater than the critical angle and, thus, be totally reflected. In fact, most fibers have a varying refractive index to allow more light to be guided along the fiber through total internal refraction.