The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, and for grading wool by the diameter of the fibres. The width of a single human hair ranges from approximately 20 to 200 μm. The longest human chromosome is approximately 10 μm in length. Around 100 μm (0.1 millimeters) is the smallest thing visible to the naked eye.