What is Optical activity, specific rotation?

A substance shows optical activity if it rotates the plane of polarisation of linearly polarised light passing through it. Seen by an observer facing the light, a dextrorotatory substance, written $(+)$, rotates it clockwise, a laevorotatory one, $(-)$, anticlockwise. The specific rotation is $[\alpha] = \alpha/(\ell c)$, where $\alpha$ is the measured angle in degrees, $\ell$ the path length in decimetres and $c$ the mass concentration in $\mathrm{g}/\mathrm{mL}$; it depends on the wavelength (usually the yellow sodium line), the temperature and the solvent.