The One Awesome Fact About Lasers That You Need to Know

When you're idly playing with a pen laser, as I'm sure many of you do, you may notice that the light it produces is strangely grainy, with light and dark patches. This is called laser speckle. Understanding it is key to understanding why lasers are awesome.

Laser speckle looks like a pattern projected over a wall, but it's not. One of the better proofs of this is the fact that people with imperfect eyesight should be able to see the speckle pattern even if their glasses are off. The wall, or nearly anything you shine laser light at, has small imperfections. These catch and reflect the light waves from the laser which up until then have been traveling in perfect sync with each other. They reflect the waves at slightly different angles. Some waves of light have to travel into a depression while others hit a mound, and so they're also projected slightly out of sync. This causes peaks and troughs of different waves to overlap. In some places the overlap cancels the waves out while in other places emphasizes them - creating light and dark patches. Again, every part of the wall is illuminated, so you aren't seeing light and dark spots on the wall. You're seeing the interference of waves traveling between the wall and your eye.

This means that your eyes are also key players in the game. A quick motion of your head while looking at a speckle pattern can tell you whether you're nearsighted or farsighted. Nearsighted people, if they move their head while looking at a speckle pattern, will see the laser speckle move the opposite direction as their motion. Farsighted people will see it move in the same direction. Light entering the eye makes a kind of an "X" shape as beams are bent towards each other by the lens in the eye. Ideally, your retina - the part that reports what it sees to the brain - should be at exact intersection of the two lines in the X, and the world will be in focus. If you are nearsighted, your retina is a little behind the intersection. If you are farsighted, your retina is a bit in front of it. Since the light that is on the left side of the X crosses over to the right side after the intersection, you will literally see the world move in different directions, depending on if you are near or farsighted.

The speckle test is a remarkably good vision test. (People with perfect vision don't see the speckles move at all.) It can even track the minor changes between far and nearsightedness that we experience throughout the day.

But speckles aren't only useful for vision problem diagnosis. They can also be helpful when designing holograms. Since we've realized that the texture of the surface on which a laser shines can set up an interference pattern for light and dark, sculpted textures should be able to set up images that have light and dark, and are always in focus. What a difference a dot makes.

Source: The Exploratorium twice.