Compared to the speed of light in air, the speed of light in water is ______.

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Multiple Choice

Compared to the speed of light in air, the speed of light in water is ______.

Explanation:
Light slows down when it travels through a material because the medium has a refractive index that affects how fast light can move. The speed in a medium is v = c/n, where c is the speed in a vacuum and n is the refractive index. Water has a higher refractive index (about 1.33) than air (about 1.0003), so light in water moves at roughly c/1.33, about 0.75c, while in air it is nearly c (0.9997c). Therefore, light travels slower in water than in air. The other options don’t fit because they would require water to have a smaller index, the same index, or a variable condition, which isn’t the case here.

Light slows down when it travels through a material because the medium has a refractive index that affects how fast light can move. The speed in a medium is v = c/n, where c is the speed in a vacuum and n is the refractive index. Water has a higher refractive index (about 1.33) than air (about 1.0003), so light in water moves at roughly c/1.33, about 0.75c, while in air it is nearly c (0.9997c). Therefore, light travels slower in water than in air. The other options don’t fit because they would require water to have a smaller index, the same index, or a variable condition, which isn’t the case here.

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