question: Because optics fibers are thin, entering light may strike the inside surface at greater than the critical angle, requiring attention to what?
distractor3: wavelength
distractor1: reflective point
distractor2: electromagnetic index
correct_answer: refractive index
support: Fiber Optics: Endoscopes to Telephones Fiber optics is one application of total internal reflection that is in wide use. In communications, it is used to transmit telephone, internet, and cable TV signals. Fiber optics 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 (See Figure 25.14. ) The index of refraction outside the fiber must be smaller than inside, a condition that is easily satisfied by coating the outside of the fiber with a material having an appropriate refractive index. In fact, most fibers have a varying refractive index to allow more light to be guided along the fiber through total internal refraction. Rays are reflected around corners as shown, making the fibers into tiny light pipes.