[mcq] Light travels at speed v_w in water and at speed v_g in glass, where v_g

Question

[mcq] Light travels at speed v_w in water and at speed v_g in glass, where v_g < v_w. Which expression correctly gives the refractive index of glass with respect to water?
(A) v_g / v_w
(B) v_w / v_g
(C) (v_w + v_g) / 2
(D) v_g × v_w

Accepted Answer

## Model Answer **(B) v_w / v_g** The refractive index of medium 2 w.r.t. medium 1 = speed in medium 1 ÷ speed in medium 2. So, $n_{gw} = \dfrac{v_w}{v_g}$. Source: Chapter 9, Section 9.3.2 --- ## Explanation The key formula from the textbook (Eq. 9.5) is: $n_{21} = \dfrac{v_1}{v_2}$ — speed in the **first** medium divided by speed in the **second** medium. Here, glass is medium 2 and water is medium 1, so the answer is $v_w / v_g$. Since $v_g < v_w$, this value is greater than 1, which makes sense as glass is optically denser than water. Avoid the common error of inverting the ratio.

Science (086) — AI-generated practice question