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Wikipedia describes solar sailing as

a form of spacecraft propulsion using a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to high speeds.

I understand the part where ejected gasses bump into the sail pushing the spacecraft. On the other hand, I don't understand how light can do this, since light has no mass.

How does that work? Does this mean that if I have a mirror balancing on a needle I would be able to push it over with my flashlight?

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I think you are really asking "how can light deliver an impulse to the sail". The answer is that although light has no mass it does carry momentum. When light is reflected off the sail, conservation of momentum requires that the sail changes momentum by twice the momentum of the light. The extra kinetic energy of the sail comes from the red shift of the reflected light.

This question has several answers that discuss the momentum of light in some detail.

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  • $\begingroup$ So this would mean I would be able to push the mirror off balance with a flashlight? $\endgroup$
    – user17615
    Commented Mar 20, 2013 at 15:15
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    $\begingroup$ Yes, though the mirror would need to be exquisitely finely balanced! $\endgroup$
    – John Rennie
    Commented Mar 20, 2013 at 15:34
  • $\begingroup$ @MichielT: in principle, yes. In practice, the idea of a solar sail is to produce a small force over a long period of time, so you'd have to shine that flashlight for quite a while to have a measurable effect. $\endgroup$
    – Zo the Relativist
    Commented Mar 20, 2013 at 15:34
  • $\begingroup$ @MichielT: given the link I mentioned in my answer you should be able to estimate the force produced by your flashlight. $\endgroup$
    – John Rennie
    Commented Mar 20, 2013 at 15:37
  • $\begingroup$ @John Rennie: I dont have a background in physics, so all the formulae are jibber jabber to me. But find the concept very interesting! $\endgroup$
    – user17615
    Commented Mar 20, 2013 at 15:40
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solar sailing work on the principal impulse applied on the body by a moving photon on a reflective surface. If the ratio of the force applied by the photon which is directly proportional to the area of the reflecting surface and intensity of the light to the weight of the space craft is large enough it would accelerate the spacecraft and further you can go to following link it would help you through interesting videos (http://www.youtube.com/results?search_query=solar+sail+spacecraft%2F+discovery&oq=solar+sail+spacecraft%2F+discovery&gs_l=youtube.3...1334.10855.0.11007.34.27.1.6.7.1.497.5586.7j7j5j3j4.26.0...0.0...1ac.1.BqfM0pGzoz8)

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Light is an electromagnetic wave. When light arrives at a solar sail the electric and magnetic fields that make up the light accelerate the electrically charged partciles in the atoms of the sail, after all, that's what electric and magnetic fields do. If these particles are accelerated their momentum is changed and so for momentum to be conserved, that momentum must come from somewhere. It must come from the light. Computing the details of which direction those forces act in requires some mathematics, but I hope this will convince you that in principle light can 'push' something.

So ultimately a solar sail works for exactly the same reason that a magnet can push another magnet, because charged particles in electric and magnetic fields experience a force.

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