Assume a visible photon of $3 \mathrm{eV}$ energy is absorbed in one of the cones (light sensors) in your eye and stimulates an action potential that produces a $0.07$ volt potential on an optic nerve of $10^{-9} \mathrm{~F}$ capacitance.

Calculate the charge needed.
$Q=V C=0.07 \times 10^{-9}=7 \times 10^{-11}$ Coulomb.