Laser speckle—the dazzling pattern of light and dark spots you see here—arises from a phenomenon known as wave interference.
Light waves emerge from a laser coherently—that is, with the crests of all the light waves aligned. But when laser light strikes a rough surface, such as the frosted surface of a light bulb, the reflected waves become scattered, and the crests of these scattered waves may either line up or not.
In places where crests do coincide, the waves add together, or interfere constructively, to make bright spots. In other locations, light waves may cancel, or interfere destructively, creating dark spots. These light and dark spots are collectively known as laser speckle.
This speckle pattern will be sharply in focus no matter where you focus your eyes, and their apparent motion when you move your head will depend on the acuity of your vision: Nearsighted people will see the speckle move opposite to their head motion, while farsighted people will see it move in the same direction. The worse your vision is, the faster the dots will seem to move.
The apparent motion of the speckle pattern arises from the way images form in each viewer’s eye. When a nearsighted person looks at the speckle-covered light bulb, they tend to focus their eyes slightly in front of the bulb, which focuses the bulb’s image slightly behind the retina. Then, when they move their head from side to side (using the position of the bulb to identify a reference plane), the speckle seems to move in the opposite direction. You can model this effect by placing one finger in front of the other, directly in your line of sight. As you shift your head to the right, the finger in front seems to shift to the left.
Unlike a nearsighted viewer, a farsighted person will focus on a plane slightly behind the bulb. That means the bulb will come into focus slightly in front of the retina. Then, when the viewer moves their head from side to side, the speckle pattern will seem to move along with them.
Those few rare people with excellent eyesight will see no motion in the laser speckle at all when they move. For them, the speckle dots will be imaged on the retina along with the bulb, so they’ll see no relative motion between them.
The apparent motion of laser speckle serves as a very sensitive vision test. Even people with excellent vision may test as slightly nearsighted or farsighted on any given day or at different times of day, as slight changes in intraocular pressure change the shape of the eyeball.