Laser Speckle

Warning: Never look directly into a laser, or point a laser into another person’s eyes.

1. Set up the frosted bulb securely on a table. (Don’t try to hold the bulb in your hand to do this activity; even small motions will destroy the effect.)
2. Turn on the laser, using the tape or binder clip, if necessary, to keep it on (see left photo below). Mount or prop up the laser so that it shines into the bulb from the side opposite your viewing location (see right photo below). Make sure the laser is also firmly mounted—not handheld—and pointed straight into the light bulb. The frosted bulb will scatter the laser light, which will prevent it from passing through the bulb to your eyes. You should see the frosted light bulb glow with laser light.

    

Stand back 3 to 6 feet (1 to 2 m) from the bulb. Look closely and you’ll see many tiny bright and dark spots in the light. This is called laser speckle.

Move your head to the right. Which way does the speckle pattern move? Now move your head to the left, which way does the speckle pattern move? You may see the speckle move in the same direction as you move your head, or you may see it move in the opposite direction.

Move farther from and closer to the bulb and try again. You may get different results at different distances.

If you wear glasses, trying removing them. You’ll see the speckle as sharp points, even without corrective lenses. The speckle pattern may move in opposite directions with and without your glasses.

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.

If you want to present this Snack to a large group, try shining the laser through a converging lens onto a white screen, producing a disk of light. A stronger lens and greater distance from the lens to the screen will produce a larger but dimmer disk. A 4-inch-diameter (10-cm-diameter) disk (roughly fist-sized) will be sufficient for individual viewing, but you’ll need a larger disk for classroom observing. Experiment to find a size that works for you. If you choose to project the speckle in front of a class, you’ll need a 1-milliwatt or stronger laser.

When you do this activity with a group, each observer will make one of two opposite observations, depending on whether they’re nearsighted or farsighted. They’ll then be faced with a decision: Do they report what they see (even if they’re the only one who sees it), or do they go along with the group? This models the behavior of scientists in the real world.

As you guide your group, you can make the point that the advancement of science depends upon accurate observation and honest reporting. You might also start a discussion on why some observers might disagree with a majority opinion, and whether the majority opinion should ever be considered the “correct” opinion.