Problem 5.199

Experience shows that a body irradiated with light with circular polarization acquires a torque. This happens because such a light possesses an angular momentum whose flow density in vacuum is equal to $M=I / \omega,$ where $I$ is the intensity of light, $\omega$ is the angular oscillation frequency. Suppose light with circular polarization and wavelength $\lambda=0.70 \mu \mathrm{m}$ falls normally on a uniform black disc of mass $m=10 \mathrm{mg}$ which can freely rotate about its axis. How soon will its angular velocity become equal to $\omega_{0}=$ $=1.0$ rad/s provided $I=10^{\circ} \mathrm{W} / \mathrm{cm}^{2} ?$