Wavelength indicated the distance from one crest to another, or from one trough to another, of an electromagnetic wave like light. Crest is the highest point of the wave whereas the trough is the lowest. Since wavelength is a distance, it is measured in units of lengths such as metres, centimetres, millimetres, nanometres, etc. The wavelength of the light determines its color. The human eye can perceive light with wavelengths ranging from about 380 nm (violet) to 780 nm (red). Since optical properties of a substance depend on the wavelength of the light they were determined with, it is extremely important to use exact wavelengths for measurement. All SCHMIDT + HAENSCH polarimeters, refractometers and photometers work with precise wavelengths, thus avoiding unnecessary errors.