  We argue that the detailed analysis of the optical response in cuprate
superconductors allows one to verify the magnetic scenario of superconductivity
in cuprates, as for strong coupling charge carriers to antiferromagnetic spin
fluctuations, the second derivative of optical conductivity should contain
detectable singularities at $2\Delta +\Delta_{\rm spin}$, $4\Delta$, and
$2\Delta+2\Delta_{\rm spin}$, where $\Delta$ is the amplitude of the
superconducting gap, and $\Delta_{s}$ is the resonance energy of spin
fluctuations measured in neutron scattering. We argue that there is a good
chance that these singularities have already been detected in the experiments
on optimally doped $YBCO$.
