A interesting phenomenon about fingerprints and liquids

In summary, during lunchtime, the speaker discovered a phenomenon where when looking from above at the inner walls of a glass while holding it, they could see a clear image of their fingertips. This does not happen when using other materials. The speaker wonders if this is due to total internal reflection. The reason for this is because when fingers touch the glass, the similar refractive indices of glass and water cause total internal reflection to not occur. The speaker also notes that refraction causes light to bend downwards, preventing objects from being seen through the water. Damp cloth can reveal fibers on the glass, but not when dry.
  • #1
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Hello,I just discover a interesting phenomenon at lunch time when I drink a glass of water:

When I hold a glass in my hands, If I look from above at the innerwalls of the glass, we can see a very bright and clear image of patterns on your fingertips through the glass. But when I use other meterials, we can only see the innerwalls of the glass from the top of the glass, but can't see the material.

How do the phenomenon happens? I wonder it is because of the total reflection.

I'm very eager to understand it!

Thank you very much!


With best regards,


Julia
 
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  • #2
Welcome.
Yes, it is a TIR thing.
TIR works at quite steep angles for glass into air - hence the inside possibly looks shiny or you see the bottom of the glass perhaps. This is also why bubbles look shiny. When your fingers (with slightly damp surfaces) touch the glass, TIR no longer happens at that angle because the two media (glass and water) have similar refractive indices (relates to densities). You see only what comes through the glass and not the internal reflection. It will only happen at very oblique angles. OH yes - why don't you see through to the outside of the glass through the surface of the water? Refraction causes the light coming in from the side to be bent' downwards' to below the lip of the glass. If you put your head very low (almost sideways), you can actually see some objects on the table through the water. (I just went out to the kitchen to confirm this - how sad am I?)
If you put damp cloth against the glass you will probably see the fibres but not when it's dry.
Enjoy your next drink.
 
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  • #3
Thanks you very much!
That really helps, I'm very appreciate about it!
 

1. What is the science behind fingerprints?

The unique patterns and ridges on our fingertips are formed during fetal development and remain unchanged throughout our lives. These patterns are determined by genetics and are influenced by environmental factors such as pressure and movement.

2. How do liquids affect fingerprints?

Liquids can temporarily alter the appearance of fingerprints by smudging or diluting them. However, the unique patterns and ridges will still be present and can be revealed through techniques such as dusting or chemical processing.

3. Can fingerprints be erased or changed?

No, fingerprints cannot be erased or changed. The ridges and patterns are formed in the deeper layers of the skin and cannot be altered without causing damage. In rare cases, severe burns or injuries can result in changes to fingerprints, but this is not a common occurrence.

4. Why are fingerprints considered a reliable form of identification?

Fingerprints are considered a reliable form of identification because they are unique to each individual and cannot be easily replicated. Even identical twins have different fingerprints. The chances of two people having the same fingerprints are incredibly low, making it a highly accurate form of identification.

5. Can fingerprints be used to determine a person's gender or race?

No, fingerprints cannot be used to determine a person's gender or race. Fingerprints are formed based on genetics and are not influenced by factors such as gender or race. Additionally, fingerprint patterns and ridges are not specific to any particular gender or race.

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