Does a light sail move forward if the light source is originated from the craft? I recalled an article I read that light sail will still push forward if the light shine onto the sail from the craft.  If the article is correct, I am curious if Newton Third Law is violated.
 A: On turning on the light source the craft will be accelerated backward until the light hits the sail. If the sail is absorptive, the acceleration will cease when the light starts hitting the sail, and the craft will come to rest. When the light is deactivated, the reverse shift will happen, and both will be immeasurably tiny.
If the sail is reflective, when the light starts reflecting off of it the net acceleration will be in the forward direction. The problem is that the magnitude of the acceleration is guaranteed to be less than if you had directed the light out the back of the craft to begin with, if possible. If not, then the sail is basically acting like the collimating reflector on a flashlight.
The reason that Newton's law isn't violated is because the sail is reflecting the light, sending it off in the direction opposite the way the craft is accelerating.
A: If you shine a (very) strong light beam on a light sail from a space craft where it is fastened and its reflection leaves the space ship, the space craft will move. But you don't need the light sail for this effect. You can simply point the light beam opposite to the direction you want to move and you will propelled by the reactive force of the emitted photon momentum stream per unit time $$F=R\frac{\hbar \omega}{c}$$ where $R$ is the emitted number of photons per second. Photon rocket propulsion, e.g. by matter-antimatter annihilation, has been proposed by the Austrian aerospace engineer and space travel visionary Eugen Sänger.
