We first apply Snell's Law to the upper surface of the fiber and find the condition for total internal internal reflection.
Total internal reflection occurs when Snell's Law fails because of the fact that cannot be larger than . If we take medium to be the optical fiber and medium to be free space, then and . So, Snell's Law will fail when or Now, from the right triangle in the diagram, we see that an application of Snell's Law to the leftmost surface of the fiber gives So, light will stay in the fiber when So, the condition for total internal reflection is or Therefore, answer (B) is correct. |

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