= Femtometre
{title2=$1\,\mathrm{fm}=10^{-15}\,\mathrm m$}
{wiki}
A <femtometre> is $10^{-15}$ <metre>, written $\mathrm{fm}$. It is a convenient <unit of measurement> for the dimensions of an <atomic nucleus>, rather than the much larger dimensions of a <chemical atom>. For example, a <length> of $5\,\mathrm{fm}$ is $5\times10^{-15}\,\mathrm m$.
<Scattering> probes such small <lengths> through <momentum transfer> rather than a miniature mechanical ruler. The relation between <wavelength> and <momentum> explains why increasingly short-distance structure requires increasingly large <momentum transfer>.
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