Seeing Things

You've probably split sunlight into a rainbow of colors with a prism. Maybe you've also split sunlight with a spray of water from a garden hose or even with a CD. But have you ever tried splitting light with your fingernail?

That's right. Your fingernail. On a bright, sunny day, if you look at sunlight reflecting off your fingernail at just the right angle, you might see a dancing pattern of colorful speckles.


A sunlight speckle pattern photographed by Stewart McKechnie.

A prism, garden hose, and CD all separate the colors into rainbows. Usually, you get speckles only with a laser. A laser, such as a pointer that creates a red spot on a wall or some other surface, produces light of a single color (or wavelength). These waves are also locked in step so that they overlap exactly. Scientists describe them as coherent. When this light reflects from a surface that isn't completely smooth, the waves no longer line up perfectly. In some places, the reflected waves cancel each other to create a dark spot. In other places, they reinforce each other to create a bright red spot. The result is a rapidly shifting pattern of tiny red and black dots.

Sunlight, unlike laser light, is made up of many different wavelengths and these waves are not coherent. Nonetheless, it's possible to see a sunlight speckle pattern under the right conditions, says scientist Stewart McKechnie of ITT Industries in Albuquerque, New Mexico. To see sunlight speckle, he says, you might have to experiment a bit. You have to look at your fingernail from the direction in which it would reflect sunlight if it were a mirror (now there's a concept). And you have to look at it as closely as you can without losing focus. Also, if you have astigmatism this won't work for you (sorry).

So what's happening? It turns out that to scatter sunlight into the speckle pattern, a surface must be rough but not too rough. A fingernail has just the right amount of roughness for you to see the speckles.

Besides, now you have a nice excuse for going outdoors on a bright, sunny day.

Let us know what happens when you try this experiment—even if you fail.

It would be much easier to see sunlight speckle if you lived on Pluto and the sun was just a tiny spot in the sky.

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Muse, July/August 2005, p. 19.