Medium
The refractive index of glass is 1.5 and the speed of light in vacuum is 3 × 10⁸ m/s. What is the speed of light in glass?
Explanation
Explanation
Step 1: Refractive index n = (speed of light in vacuum) ÷ (speed of light in the medium) = c ÷ v.
Step 2: So v = c ÷ n = (3 × 10⁸) ÷ 1.5.
Step 3: v = 2 × 10⁸ m/s.
Step 4: Sense check: light always slows down in a denser medium, so v must be less than 3 × 10⁸ m/s. The option 4.5 × 10⁸ is impossible.
Answer: 2 × 10⁸ m/s.
PRO TIP
Re-check your working by substituting the answer back into the original question.