vacuum

Meaning: A vacuum is a region with no matter, so an ideal vacuum has an absolute pressure of 0 Pa. Real vacua, such as outer space or a laboratory chamber, have very low pressure but not exactly zero. With no particles present, there is no medium for mechanical waves, no air resistance, and no conduction or convection.

Equation: n = c/v, where n is the index of refraction (unitless), c = 3.00 × 10⁸ m/s is the speed of light in vacuum, and v is the speed of light in the medium (m/s). In vacuum v = c, so n = 1.

On the MCAT: Light and other electromagnetic waves cross a vacuum, but sound does not. A hammer and a feather dropped in an evacuated chamber have the same acceleration g and land together. Heat from the Sun reaches Earth by radiation, since conduction and convection need matter. A vacuum layer in a thermos cuts conduction and convection. In Coulomb's law, k = 1/(4πε₀), where ε₀ is the permittivity of free space (vacuum).

Common confusion: Students think a vacuum means no gravity or no fields. A vacuum removes matter, not fields. Gravitational, electric and magnetic fields still act across it, so objects keep their weight and charges still exert forces.