refractive index
Summary
The refractive index n = c/v compares the speed of light in a vacuum with its speed in a medium, so n is unitless and at least 1. A higher n means slower light. A ray entering a higher-n medium bends toward the normal.
Detail
Meaning: The refractive index n measures how much a medium slows light. At a boundary the frequency stays the same, so a lower speed means a proportionally shorter wavelength. A vacuum has n = 1, air is very close to 1, water is about 1.33 and glass about 1.5. Higher n means optically denser. Entering a higher-n medium bends a ray toward the normal; entering a lower-n medium bends it away.
Equation: n = c/v, where c = 3.00 × 10⁸ m/s is the speed of light in vacuum, v is its speed in the medium (m/s), and n is unitless. Snell's law: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal.
On the MCAT: Passages give n for two media and ask which way a ray bends, how speed or wavelength changes, or whether total internal reflection occurs, as in fiber optics. The critical angle follows sin θc = n₂/n₁ and exists only when light goes from higher n₁ to lower n₂.
Common confusion: Frequency stays constant across a boundary; speed and wavelength change. Angles are measured from the normal, not the surface. Optically denser refers to n, not mass density.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway, Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.