refractive indices

Meaning: The refractive index (plural: indices) measures how much a material slows light. Frequency stays the same across a boundary, so a lower speed means a shorter wavelength: λmedium = λvacuum/n. A higher n is called optically denser. Vacuum is 1, air is nearly 1, water is about 1.33 and glass is about 1.5.

Equation: n = c/v, where c = 3.00 × 10⁸ m/s is the speed of light in vacuum, v is the speed in the medium (m/s), and n is unitless. Refraction follows Snell's law: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. Critical angle: sin θc = n₂/n₁, which exists only when n₁ > n₂.

On the MCAT: A passage gives indices and asks which way a ray bends. Entering a higher n, light slows, its wavelength shrinks, and it bends toward the normal. Entering a lower n, it bends away from the normal. Total internal reflection needs light going from higher to lower n at an angle beyond θc. Other questions ask for the speed or wavelength of light in a material.

Common confusion: Students say the frequency changes, or they measure angles from the surface. Frequency is set by the source and stays constant, while speed and wavelength change. Angles in Snell's law are always taken from the normal.