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The refractive index of water is 4/3. What is the speed of light in water?

Asked in GSEB Board August 2020 · Snell's law, slabs and apparent depth

• (1) 3×10⁸ m s⁻¹
• (2) 2.25×10⁸ m s⁻¹
• (3) 4×10⁸ m s⁻¹
• (4) 2×10⁸ m s⁻¹

Step-by-step solution

Why the other options are wrong

• (1) 3×10⁸ m s⁻¹ is the speed in vacuum. Light is slower in every material medium, which is what a refractive index above 1 means.
• (3) This comes from multiplying by 4/3 instead of dividing. Since n=c/v with n>1, the speed in water must be less than c — nothing travels faster than light in vacuum.
• (4) 2×10⁸ m s⁻¹ is c/(1.5): the refractive index of glass used in place of water's 4/3.

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