Use our handy dandy guide below to find out! High and Low Pressure Areas The ratio of the speed of light in a vacuum to that in the medium at hand is called the index of refraction, and is usually expressed using the formula N = (1-n)*, where n is the index of refraction and N is an integral number used to express it. In this chapter you Mercury barometer is an accurate instrument and is used as a standard. A common value for N is, but can vary between and More than just plots of raw weather r