Now look more closely at the critical region where the laser should come to a focus at the surface of the disk (which is now shown by a thin semitransparent square).
Convex lenses have a key property: If light enters the lens as a parallel beam, it will be focused into a single point. This "focal point" is fixed distance away from the lens. Here we represent this "focal length" as the first green arrow.
It also works in reverse: Light that leaves the focal point and enters the second lens is then bent back into a parallel beam.
