2. Question:
Fill a glass with a liquid and hold it in
your hands. If you look from above at
the inner walls of the glass, you will
notice that the only thing visible
through the walls is a very bright and
clear image of patterns on your
fingertips.
Study and explain this phenomenon.
4. Understanding the fingerprint
Friction ridges & furrows
Secretion of sebum & sweat oil
Impressions left on surfaces are from
secretion from ridges.
Friction ridges
6. Preliminary investigation
A ‘normal' finger produces a much
clearer image compared with a finger
scrubbed clean (no oil).
Fingers & objects dipped in oil are fully
visible
Oil plays a key role
7. Preliminary investigation
Ridges can be seen from all angles
Objects placed behind glass produces
no image, unless viewed at a low
enough angle
Geometry of light path is a factor.
However, when object is pressed onto
glass with sufficient pressure, some
image is observed.
Contact point is also a factor
9. Possible model: [1]
We see ridges simply
because they are
pushed against the
glass?
From preliminary
investigations, we know
that no image is seen
when no oil is involved.
ridge
furrow
GLASS FINGER
10. Possible model: [2]
Finger not in contact
with glass.
Frustrated total
internal reflection
(FTIR) occurs?
Air
gap
13. Possible model: [3]
oil Gap flooded with oil.
From preliminary
investigation, we know
the entire finger would
be seen, not just the
ridges if this is the
case.
14. Theory:
Higher refractive index of oil at ridge
causes light rays experiencing total
internal reflection (TIR) in glass medium
to instead enter oil at oil-glass interface.
hits skin at ridge and randomly
scatters
Meanwhile, there is air gap at furrow
◦ Light reflected through TIR
◦ Any light scattered by furrow will exit at
angles not visible to the observer.
15. Water
nwater = 1.33
θC,water-glass = 61.0°
θC,air-glass
= 41.1°
Glass
nglass
=
1.33
Schematic diagram:
Light sources for scattering
Finger
placed
against
glass
21. Observations
Dark skin coloured lines seen are
ridges.
Image of furrows are not seen. They
appear as bright lines that take on
colour of surrounding light
Bright lines are
furrows Dark, skin
coloured lines
are ridges
22. Explanation
Why ridges are skin coloured:
Subsurface scattering occurs
◦ Light will enter epidermis at skin.
contains pigment melanin.
Melanin causes a brown-black coloration and
scatters light strongly in the forward direction.
◦ Thus the image of ridges seen are skin
coloured.
23. Explanation
Why furrows appear as bright lines:
◦ As explained by model
◦ Light scattered off furrow leaves water at
very low angles not observable
◦ Observer at higher angles sees light
reflected off glass.
◦ Thus furrow do not appear as image, but
are seen as bright lines.
25. Calculations: object in contact
against glass, e.g. ridge.
For object to be seen:
1. Angle of incidence at glass-water
boundary must be < θC, glass-water
2. Angle of incidence at water-air boundary
must be < θC, water-air
Combining these 2 conditions,
◦ Scattered light must hit water-glass
interface @ 36°< i <61°.
◦ Image can be seen from anywhere
above water level. verified by
experiments
27. Why cannot we see objects placed
behind the glass unless viewed from a
very low angle?
This includes the furrow
Calculations: object behind glass
not in contact
Low angles:
can becomes
visible
High angles:
Glass acts
like a mirror
28. Calculations: object behind glass
not in contact
For object to be seen:
1. Angle of incidence at glass-water
boundary must be < θC, glass-water
2. Angle of incidence at water-air
boundary must be < θC, water-air
3. Angle of refraction at air-glass
boundary must be < θC, air-glass
Derived from principle of reversibility
Same as
conditions
for objects
in contact
29. Calculations: object behind glass
not in contact
Combining these 3 conditions,
◦ Scattered light must hit water-glass
boundary @ 36°< i <41°.
◦ Light hits water-air boundary at
43°< i <48°.
◦ ‘Visible zone’ derived to be approx.
below 25° above horizontal
32. Conclusion
Model to explain:
◦ Why we can see ridges at any angles
Oil and random scattering
◦ Why we cannot see furrows / objects placed
behind glass at high angles (>25)
Scattered light does not reach eyes
Visible range for objects is at angle
below approx. 25° above horizontal
Why can we see fingerprint clearly ridge and furrows
Why can’t we see stuff behind glass at beyond certain high angles
Here is a picture of the phenomenon. When we press our fingers to a glass of water and view it from the inner walls, we see patterns of bright and dark lines showing the pattern of our fingerprints. However, any objects behind the glass seem to become ‘invisible’. We shall attempt to explain this phenomenon.
First, we must understand our fingertips. ….. For this discussion we will simply call this mixture oil. Furrow do not make contact with surface
We know this because we have confirmed that contact point is a factor, and it is obvious that the ridge will contact surface instead of furrow. It is also know in forensics that prints left behind are from ridges.
We know that this is not true, as no image is seen when there is no oil present. Hence this cannot be the case.
FTIR occurs such that an evanescent wave, or a wave with no energy, travels out of air-glass interface when there is TIR. This wave then travels to the third medium that is the finger, and then scatters off it as visible light carrying energy.
Again, we know that this is not entirely the case, as we leave prints behind when we touch surfaces.
If there is no contact, we wouldn’t leave prints on surfaces.
We also know that this is wrong, as preliminary data shows that when the entire interface is flooded with oil, the entire finger can be seen, and not just at the points of contact. Also, if this is the case, we would not be able to leave behind clear fingerprints, but a giant smudge. (this could be a major headache in forensics… XD)
Our theory is a mixture of the three models above.
This diagram shows the light sources that allow scattering to occur. Since the finger is placed against the glass, light from behind the finger will not be a source. Light can either be from the top, right side or bottom.
In case of TIR, When air pocket is hit, any light rays hitting boundary are at angles larger than critical angle. Hence we will not see furrow.
(Principle or reversibility thus the same can be said for light from the top.)
It is possible for some light coming in horizontally at small angles to enter furrow. These light rays, however, will not be seen by observer after scattering.
Light scattering off furrow at large angles will be absorbed by ridge and re-scattered.
Rays that manage to eventually travel into water will either experience TIR at water-air boundary or emerge at very low angles. This will be validated by calculations later on.
Hence, scattered light off ridges are not observable, and ridges appear as bright patterns which are formed from surrounding light.
Due to the high refractive index of oil, light rays previously undergoing TIR in glass will enter oil medium at glass-oil boundary. It then scatters randomly at the uneven skin interface. The randomly scattered rays are then seen by the observer.
Light rays coming in horizontally are also freely scattered.
From observation, we have seen …
Attempt to quantify observations through calculations.
(The oil found on the skin surface is a complex mixture of sebum, lipids and sweat. The amount and composition of this varies with gender, age, and race. It is found to be around 1.5 according to literature.)
Due to only certain angles of light hitting glass in the so-called visible range, this explain why image of ridges seen are much less bright.
(Rays beyond 61 at glass will TIR; rays lower than 36 will TIR at water surface)
We have done an experiment find this angle as well. A camera was shifted such that an image of the can behind tank is just visible. The angle of elevation is found to be approx 28°.
