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Re: barrier penetration, evanescent waves



"... But a finger has
just the right combination of compliance and texture
so that the ridges of the fingerprint absorb the
evanescent wave (while the intervening valleys don't),
resulting in a very-high-contrast image of the fingerprint."

I forgot to mention that this is the method for fingerprinting (a digital
camera views the ftir image and sends it to the DOJ)

Before I do my demo. (semi-quantitative) for the HS Physics class, the
instructor has passed out glass prisms for the students to see for
themselves.

bc who was finger printed this way as a requirement for a teaching
credential.

"John S. Denker" wrote:

Bernard Cleyet wrote:

A model for barrier penetration is FTIR (frustrated total internal
reflection). The model is exact in that the equations are of the same
form (obvious, they're both described by the same wave equation).

Yup.

For visual frequencies measurements are very difficult!

But qualitative results are not out of reach: I have
been fascinated since childhood by what happens when
you touch your finger to the side of the fish-tank.

Except where touched, the side-wall is verrrry shiny
due to total internal reflection. But a finger has
just the right combination of compliance and texture
so that the ridges of the fingerprint absorb the
evanescent wave (while the intervening valleys don't),
resulting in a very-high-contrast image of the fingerprint.

The length-scale involved (the amplitude of the fingerprint
ridges) provides some semi-quantitative information about
how far the evanescent wave can penetrate. As BC points
out, it would be hard to make this into a quantitative
measurement. But at least it gives the students a
"hands on" picture of the length-scales involved.