Imagine running on a smooth paved road (air) versus running through knee-deep water (water). You naturally slow down in water because of greater resistance. The refractive index value tells you how many times more resistant water is to light's travel compared to open air.

Remember 'NAV': N = Air / Vacuum speed over Medium speed ($n = c / v$). Higher $n$ means slower speed in that medium!

When light enters an optically denser medium like water from air, its speed decreases due to interaction with the atoms of the medium. The absolute refractive index is given by Snell's relation $n = \frac{c}{v}$, where $c$ is the speed of light in vacuum/air ($3 \times 10°8\text{ m/s}$) and $v$ is the speed of light in the given medium. Rearranging this gives $c = n \times v = 1.33 \times v$, showing that light's speed in air is 1.33 times its speed in water.

Questions on Snell's Law ($n_1 \sin i = n_2 \sin r$) and Critical Angle ($\sin C = \frac{1}{n}$) frequently use $n_{\text{water}} = \frac{4}{3} \approx 1.33$ and $n_{\text{glass}} = \frac{3}{2} = 1.5$. Memorizing these fractions speeds up numerical calculations dramatically!