Have you ever gripped a drinking glass, looked down, and saw your fingerprints on the side of the glass? That mundane phenomenon is due to a crazy thing called “quantum tunneling.” It explains why your fingerprints are on a glass, and why fingerprint scans work.
The Experiment in the Kitchen
Go and get a glass of water. The glass should have smooth transparent sides. Grip it loosely and look into it. Assuming the water quality in your area is decent and the glass is clean, there shouldn’t be anything particularly interesting down there. In fact, the reflection from the light going down into the water should totally conceal your fingers. Move your head around until you get that wall of reflected light from the back of the glass. That will let you know the correct angle of sight for the second part of the experiment.
Now grip the glass tightly. Look down. You should see the ridges of your fingerprints on the glass. The hollows of your fingerprints should be bright with reflected light. The ridges, however, should be dark. No light is being reflected back to you.
Frustrated Total Internal Reflection
First, let’s look at why the light gets reflected from the wall of glass in the first place. As light moves from one medium to another, it bends. Sometimes, when the light is angled just right and the two media have just the right properties, the light is reflected totally. We see this in fiber optic cables, or other prisms. We also see it in the glass, although it’s a bit more complicated there because the light goes from water to glass to air. The point is, when the light wave hits the air it gets bounced back perfectly. When you grip the glass loosely, you aren’t seeing even the shadow of your hand on the other side of the glass.
Now let’s frustrate that internal reflection. Look at the picture of this semi-circular prism, which is reflecting light so beautifully. What would happen if we were to place another semi-circular prism near it, so the two made a complete circle? At first, nothing would happen. Press them together a bit, and that beam of light would be split. Part of it would be reflected, but part of it would go straight through the “circle.” Really press them together, and the whole beam of light would pass through the two prisms as if they were one circular prism. This would happen even though a little space would be left between them, the light would act as if the space weren’t there.
Classically speaking, that should never happen. When we study light, in prisms or water glasses, we have to factor in quantum mechanics. Light particles, or light waves, that hit a barrier — in this case the barrier of air — should bounce back. But their wave function exists, just a little bit, where classical mechanics says they can’t be. This wave function penetrates the air and, if the barrier of air is thin enough, into the material on the other side of the air. This allows the light to tunnel through that little barrier when, according to classical mechanics, it can’t.
Fingerprints on a Glass and on a Scanner
When you grip the glass tightly, the ridges of your fingerprints are so close to the glass that the light is able to tunnel through that thin barrier of air and into your fingers. Your skin is absorbing the light. The hollows of your fingerprints, on the other hand, have enough air between them and the glass that the light can’t tunnel through. It is reflected.
This is the technique often used by fingerprint scanners. When a finger is pressed against the scanner, a light from inside the scanner illuminates the finger. The ridges of the fingerprint absorb the light, because the light can tunnel through the tiny barrier of air between the ridge and the glass. The hollows of the fingerprint bounce the light back. The scanner takes note of that pattern of dark and light.