The term comes from the Greek word for dwarf: nanos or nannos. Like the prefixes centi-, milli- or deci-, nano- simply describes an order of magnitude: a nanometer (nm) is a billionth of a meter, i.e. 0.000000001 meters. By way of comparison: A human hair is 50,000 nm thick, an atom is just 0.1 to 0.3 nm and the human genetic substance DNA (desoxyribonucleic acid) has a diameter of approximately 2 nm. Things as small as this cannot be seen with the human eye and even conventional optical microscopes are not sufficient. Instead, you need special microscopes, such as scanning electron microscopes or transmission electron microscopes (SEM and TEM).
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What are nanotechnology, nanomaterials, and nanostructured materials?