Refractive index is a dimensionless number which is a measure of the speed that light travels through a medium compared with the speed of light in a vacuum. For example, the refractive index of water is 1.33 meaning that light travels 1.33 times slower in water than it does in a vacuum. Refractive index is always more than 1 and for most compounds it is between 1.3 and 1.7. It is typically measured to four decimal places as long as the substance is clear (allows light to pass through). When measured at a standard temperature (20 °C) it is, like melting point, constant for each compound. Refractive index, like melting point, is also easily altered by a small amount of an impurity .