True or false: If a medium has index n greater than 1, light travels slower than in vacuum.

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Multiple Choice

True or false: If a medium has index n greater than 1, light travels slower than in vacuum.

Explanation:
Light’s propagation speed in a medium is set by the refractive index. The phase velocity is v_p = c/n. When the index n is greater than 1, v_p is less than c, so light travels slower than in vacuum. This is why light slows down entering materials like glass or water. In isotropic, non-dispersive media, polarization doesn’t affect the speed, and for a given frequency the group velocity (the speed of a pulse) often matches the phase velocity, though dispersion can make those speeds differ with frequency. Overall, having n > 1 means the light moves slower than in vacuum.

Light’s propagation speed in a medium is set by the refractive index. The phase velocity is v_p = c/n. When the index n is greater than 1, v_p is less than c, so light travels slower than in vacuum. This is why light slows down entering materials like glass or water. In isotropic, non-dispersive media, polarization doesn’t affect the speed, and for a given frequency the group velocity (the speed of a pulse) often matches the phase velocity, though dispersion can make those speeds differ with frequency. Overall, having n > 1 means the light moves slower than in vacuum.

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