13. Optic fibers are now important for telecommunication. What is the most important physical reason for the light to be capable to travel in optic fibers? (A) Huygens's principle. (B) Total internal reflection. (C) Dispersion. (D) Brewster's law.

4. A point-like bug of mass m lies on the rim of a uniform disk of mass M that can rotate freely about its center. Initially the bug and disk rotate together with an angular velocity ω₀. Then the bug walks half way to the center of the disk. What is ratio of the new kinetic energy of the system to its initial kinetic energy? (A) (4m+2M)/(m+2M) (B) (m+2M)/(4m+2M) (C) (2m+4M)/(2m+M) (D) (2m+M)/(2m+4M)

13. Optic fibers are now important for telecommunication. What is the most important physical reason for the light to be capable to travel in optic fibers?
(A) Huygens's principle.
(B) Total internal reflection.
(C) Dispersion.
(D) Brewster's law.

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