NumericalModerate2 marks
4(ii)
A type of glass block has a refractive index of 1.8.
Calculate the speed of light in this glass. (Given speed of light in air 3 × 10⁸ m s⁻¹)
Topic: Refractive index and speed of light
Question wording verified against source paper
Answer
Exam answer
Given: refractive index of the glass μ = 1.8
speed of light in air c = 3 × 10⁸ m s⁻¹
μ = speed of light in air ÷ speed of light in the medium μ = c ÷ v
Rearranging: v = c ÷ μ = (3 × 10⁸) ÷ 1.8 = 1.67 × 10⁸ m s⁻¹
Speed of light in the glass = 1.67 × 10⁸ m s⁻¹
Explanation
Light travels SLOWER in a denser medium — that is what a refractive index greater than 1 means. So divide by μ; multiplying gives 5.4 × 10⁸ m s⁻¹, faster than light in vacuum, which is impossible and should stop you at once.
Use that as your check every time: the speed in any medium must come out LESS than 3 × 10⁸ m s⁻¹.
Keep the standard form. 3 ÷ 1.8 = 1.67, and the 10⁸ carries through unchanged.
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