Refractive Index

Refractive Index When light passes from the air into a liquid (or liquid to air) it is bent or refracted ie. its direction of travel is altered and the light ray is bent. This phenomenon can be simply illustrated by putting a pencil in a glass of water. A pencil sticking partly out of the water looks bent at the water's surface. This is because the light rays travelling from the pencil to the eye are refracted or bent when they traverse from water to glass and then again from glass to air.

Air water and glass have different densities and the velocity of light is different in each medium. When a ray of light enters a denser medium, it changes direction and enters at a smaller angle than that at which it approached the point of incidence. The refractive index is always more than 1, for light passing from a lesss dense medium (air) to a more dense (oil). The refractive index is measured by an instrument called a refractometer. Each essential oil has a refractive index within fairly narrow limits, and any deviation can indicate adulteration, poor quality or even a totally different substance.