A interesting phenomenon about fingerprints and liquids

[hchs9818]

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

 

[sophiecentaur]

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.

 

[hchs9818]

Thanks you very much!
That really helps, I'm very appreciate about it!