How To Use A Laser Pointer To Measure Tiny Things

Like a lot of people, I own multiple laser pointers. These come in handy when I do public speaking for work, but that’s only once a month or so. And with the sad passing of our dog back in December , we can’t make her chase the glowing spot around the room any more. So you might be wondering why I still keep lasers lying around.

Of course, this is not a unique situation. After all, one of the scientists who made the first working laser, Irnee D’Haenens, famously called it “A solution looking for a problem” (attribution from here). In physics, of course, the principal application of lasers has been to enable extremely precise measurements, and this can be extended to household applications, as well. With a laser pointer and a bit of ingenuity, you can make reasonably accurate measurements of things that are too small to measure with more conventional tools.

One of the canonical examples of a very small thing that you encounter on a regular basis is hair. In popular science writing, anything with a size measured in micrometers is almost inevitably compared to the thickness of a human hair, which is somewhere in the neighborhood of 100 microns.

That’s also pretty close to the limit of naked-eye vision, which makes it difficult to get a sense of finer distinctions than that. If you have hair from multiple different people, for example, it’s kind of hard to say how their sizes compare just from looking– visibility of individual hairs is only an imperfect guide, as that’s complicated by the issue of color.

You can, of course, get out some magnifying apparatus, and try to look a little closer, but that’s hard to do quantitatively. Here’s a composite image showing hairs from me, my wife Kate, and our seven-year-old daughter “SteelyKid,” whose digital microscope I borrowed to make the image:

A laser, however, offers a way to measure the thickness of a hair using a yardstick. As a demonstration, I borrowed some of SteelyKid’s Lego bricks to build both a holder for a laser pointer and a frame I could tape three pieces of hair to. Shining the laser onto the hair (as seen in the photo above) and then onto the wall of our dining room produced clear patterns of bright and dark spots, seen here:

The physics here is the wave phenomenon known as diffraction. Light waves from the laser encountering the hair can pass around it either on the right or on the left on their way to the wall. Those different paths have different lengths, and there are places on the wall where one path is half a wavelength longer than the other, meaning that the peaks of the waves that followed that path fall in the valleys of the waves from the other, and cancel out. The exact position of these dark spots depends on the distance to the wall, the wavelength of the light, and the thickness of the hair (it’s essentially a single slit diffraction pattern; the hair is kind of an inverse slit, but the math works out the same way).

The wavelength of a green laser pointer is around 532nm, and the distance to the wall and spacing between dark spots are easily measured with an ordinary meter stick, so this can be turned around to determine the thickness of the individual hairs. And I hope you’ll agree that the spacing between dark spots in these diffraction patterns is more clearly different than the thickness seen in the microscope images. Using Tracker Video to measure the spacing gives me values of (from top to bottom) 83, 107, and 89 microns for my hair, Kate’s hair, and SteelyKid’s hair. That ordering seems pretty plausible, as numerous family members have remarked that SteelyKid’s hair is sort of a cross between ours (as you would expect)– it’s straight like Kate’s hair, but closer in color and texture to mine.

So, there you have it: a reason to keep lasers around the house. If you’ve got an object that’s too small to easily measure by eye, you can get a good measurement of its size using a laser pointer, a meter stick, and the physics of diffraction.