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JAMB Physics 1979 Question 42

Question 42

Refraction of Light

The speed of light in air is 3 × 108 m s−1. If the refractive index of light from air-to-water is 4/3, then which of the following is the correct value of the speed of light in water?

  1. A.4 × 108 m s−1
  2. B.2.25 × 108 m s−1
  3. C.4/9 × 10−8 m s−1
  4. D.2.25 × 108 m s−2
  5. E.4.33 × 108 m s−1.
Show the answer and explanation

Answer: B (2.25 × 108 m s−1)

Explanation

Refractive index = speed in air ÷ speed in water, so speed in water = 3 × 108 ÷ (4/3) = 2.25 × 108 m s−1. Option D has the wrong unit.