But what happens if the disk starts to drift above or below the lens's focal point?
If the disk drops, the laser travels a longer distance between passes through the lens. So it is as if the second lens moves downward, taking its focal point with it.
But the light no longer converges at that lower lens’s focal point, so it will no longer exit the second lens as a parallel beam.
In fact, if the disk drops, the final beam converges slightly inward.
Or, if the disk rises, the final beam diverges slightly outward.
