The original direction of the (infrared) light is irrelevant, only the end result matters -- the `net force', with reflections and losses factored in. And factoring in the reflections is exactly what the article is about.
So it's not like a sailboat with a fan, more like a jet airplane with thrust reversers [1] engaged. While the original thrust is ordinarily pointed rearwards, it's redirected when the reversers are engaged and the `net force' (original thrust sans losses) is pointing frontwards.
The reason it's like a jet engine is that the engine "creates" wind - by burning fuel, the hot exhaust is the new wind. If a jet engine only moved wind (like a turbo prop), then a thrust reverser would not work.
wait, what? I'm convinced that thrust reverser would (in principle at least, construction details be damned) work with a propeller engine.
In my understanding, even if the air stream was accelerated rearwards at first, and only subsequently redirected frontward, it'd still create net force pointing frontward (decelerating one), without need for increased temperature nor extra mass from burned fuel. But I don't have solid physic background; somebody please correct me.
It may be easier to visualize it with a ducted fan. As used on some RC aircraft models, modeled after jet-engined crafts. I believe a thrust reverser would work in such a setup, minus friction and turbulent losses.
With a propeller engine, the "wind" is a side effect caused by to the primary effect of the props getting "lift" in the forward direction, similar to how the wings themselves work. The prop itself is a wing [1], which twists to create even thrust across the prop despite the speed differences between the root and tips.
Turbofans (and ducted fans) are different, as they really do produce thrust due to the particles they are throwing backwards (direct thrust).[2]
Turboprops can get reverse thrust by inverting the propellers, but adding a thrust reverser behind them would not be enough to negate the forward lift (though they could cause problems with the airflow over the props that would negate the forward lift - but it wouldn't be due to the redirected airflow).
Imagine a bent tube in a 'u' shape with a fan in the middle. Both openings of the tube point toward the front of the plane.
That's basically what a propeller engine with a thrust reverser is. And I think you can visualize how it will do nothing if you turn on the fan. The force from the air exiting and entering the tube will cancel out.
Consider it as a mass flow problem. You're ingesting a certain mass of air m at a velocity v and releasing it in the opposite direction. Assuming no losses in the U tube, the only thing that changes is the direction of v. What matters is the total momentum, mv.
If the tube were straight, mv would point out the back and the airplane would go forward. With a U shaped tube, you have instead -mv which is the same momentum, but a different direction, and the airplane moves backwards.
Now that I think of it, it might be easier to visualize as a conservation of energy problem.
Do you think so? The outward stream would probably be more directed than the inward stream, which comes in more diffuse. (Though I don't know if that matters.)
A difference in kinetic energy -- E(outflow)-E(inflow) -- of air would result in net force pointing forward (a decelerating force, if we consider a plane). Basically, we'd need the air to get accelerated forward more than rearward, producing net force.
Kinetic energy is E = (mV^2)/2
Obviously the mass of inflow is equal to the mas of the outflow. We can achieve a difference in speeds if the effective cross-section of the (forward-pointing) outlet was smaller than cross-section of the inlet.
So it's not like a sailboat with a fan, more like a jet airplane with thrust reversers [1] engaged. While the original thrust is ordinarily pointed rearwards, it's redirected when the reversers are engaged and the `net force' (original thrust sans losses) is pointing frontwards.
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[1] http://en.wikipedia.org/wiki/Thrust_reverser