= Nanometre
A <nanometre> is $10^{-9}$ <metres>. The prefix nano denotes the exact decimal factor $10^{-9}$. A length of $500\,\mathrm{nm}$ is $5\times10^{-7}\,\mathrm m$, a typical visible-light <wavelength>.<The International System of Units (SI), ninth edition, SI Brochure>[Section 3, SI prefixes]{ref}
The unit is convenient for comparing <molecules>, thin films and optical <wavelengths>. A smaller number of <nanometres> does not by itself identify a different physical regime: the relevant comparison may be with an atomic spacing, a <mean free path> or a <wavelength>, depending on the experiment.
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