Depends on what you mean by "light beam" You need to be more specific about what you need.
My guess is that you want a laser beam from one point to another. I would solve this by creating a sprite only a few pixels high with the origin in the middle of the left side. I would set the position of this sprite to the starting point. I would set the angle of the sprite to the angle between the two points, then, finally, stretch the beam to length by setting the width to the distance between the two points.
If you have trouble implementing, post screenies or a capx so we can help you figure it out