Magical fingerprints
July 14, 2011 5:37 PM   Subscribe

What is this optical effect? It looks like it's transmitting my fingerprints through plastic, but only the fingerprints.

Apologies for the quality of the photo, I only had one hand free! It's a photo of the inside of a plastic cup with water in it. My hand is wrapped around the outside; it's my fingers at the top.

As I moved my fingers back and forth different bits of my fingerprints showed up- and other bits not in direct contact with the plastic disappeared.

It was pretty weird, as if it was being projected onto the inner surface of the cup. What on earth is this? How does it happen? It seems to need the water on the inside of the cup, and maybe my fingers just slightly moist, though I'm not sure about that.
posted by BungaDunga to Science & Nature (7 answers total) 2 users marked this as a favorite
 
Best answer: I think you're seeing total internal reflection at the plastic air interface. But at the plastic-finger interface no such effect happens, so you just see your fingers. But your fingers only touch the plastic where they are ridged, so you see your fingerprints.
posted by aubilenon at 5:56 PM on July 14, 2011 [1 favorite]


Oh, to confirm this, the effect should stop happening if you look directly at your fingers through the side of the glass (instead of the oblique angle you're looking at them from above)
posted by aubilenon at 5:57 PM on July 14, 2011


Best answer: Total internal reflection. Here's a fingerprint explanation.
posted by amoeba syndrome at 5:58 PM on July 14, 2011 [1 favorite]


Response by poster: Hah, that was fast. Very cool, thanks!
posted by BungaDunga at 6:07 PM on July 14, 2011


Holy Awesome! This is the coolest thing I've learned about total internal reflection since fiber optic cable.
posted by Phredward at 7:19 PM on July 14, 2011


You may also find it interesting that this effect, frustrated total internal reflection (when something in contact with the surface disrupts the reflection, hence "frustrated") is one of the ways to make touch sensors.
posted by NMcCoy at 9:58 PM on July 14, 2011


Optical Engineer here, thirding aubilenon's explanation of TIR / Total Internal Reflection. It's the fundamental principle that allows fiber optics to work: the light beams in a long fiber may bounce back and forth millions or billions of times during their travel, so even a teeny tiny loss of reflection would obliterate the signal quickly. However, TIR is perfect: no losses at reflection. (The challenge in developing FO communication was removing absorption losses from water molecules in the glass - these were sparse, but enough to cause problems.)
posted by IAmBroom at 11:46 AM on July 15, 2011


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