fresnel lens to magnify things underwater

[knotical]

Some SB posts have suggested carrying a magnifying glass to view small marine life, but I’ve not had much luck with this. Now I know why. The Index of refraction of glass and plastic are both closer to water than to air, and when a typical lens has water on both sides, there isn’t much refraction going on. Recently I started carrying a small plastic lens I found in an office supply store. It is flat, flexible and the size of a credit card. It’s clear plastic with concentric circles etched into its surface. Costs about a buck US. This works because it does not rely on its index of refraction. Hope this helps someone else.

 

[Uncle Pug]

Well I haven't tried a frensel lense but I will. I have trapped air on both sides of a glass magnifier using lexan... but so far they have all eventually leaked and once filled with a bit~0~water became unusable.

 

[wcl]

Some SB posts have suggested carrying a magnifying glass to view small marine life, but I’ve not had much luck with this. Now I know why. The Index of refraction of glass and plastic are both closer to water than to air, and when a typical lens has water on both sides, there isn’t much refraction going on. Recently I started carrying a small plastic lens I found in an office supply store. It is flat, flexible and the size of a credit card. It’s clear plastic with concentric circles etched into its surface. Costs about a buck US. This works because it does not rely on its index of refraction. Hope this helps someone else.

Hi,

Actually, the Fresnel lens works using angled surfaces of differing indices of refraction just like any other lens.

Monsieur Fresnel had the brilliant insight to remove the useless material between lens surfaces, allowing for a much more efficient, lightweight, smaller lens for the magnification.

Here's one of many links describing the elegant solution: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/fresnellens.html .

Clever, celver, these humans!

Cheers,
Walter

 

[knotical]

The one I have is just cheap plastic. So it appears that, according to that web site, it shouldn't work. But it does work. I'm confused, but that's nothing especially new. I'll just keep using it, even if I don't understand it.

 

[pete340]

The one I have is just cheap plastic. So it appears that, according to that web site, it shouldn't work. But it does work. I'm confused, but that's nothing especially new.

My guess is that surface tension holds a thin layer of air against the grooves in the lens.

 

[kidspot]

The one I have is just cheap plastic. So it appears that, according to that web site, it shouldn't work. But it does work. I'm confused, but that's nothing especially new. I'll just keep using it, even if I don't understand it.

no - it's the same principle - just using stamped plastic instead of glass. And many more concentric rings than the ones used in lighthouses. This keeps it very flat. The same thing is used in an overhead projector - it focuses the available light into a very small area.

Neat idea that you have though - I have one of those myself but it's 8x10 inches - might be a little big to carry while diving, but the credit card sized one you suggest is a very simple and elegant solution. Plus it doesn't take up nearly the space of a larger glass magnifier and won't break as easily.

I wonder if the magnification is greater than a standard glass one and that is why it still works underwater? It's index of refraction is greater than the water's???

Aloha, Tim

 

[pete340]

I wonder if the magnification is greater than a standard glass one and that is why it still works underwater? It's index of refraction is greater than the water's???

That's possible, and easy to check: the magnification would be smaller underwater than on the surface.

 

[wcl]

The one I have is just cheap plastic. So it appears that, according to that web site, it shouldn't work. But it does work. I'm confused, but that's nothing especially new. I'll just keep using it, even if I don't understand it.

Most of modern life is beyond our understanding ... ever since we could write things down, we can accumulate way more smart stuff than any one could possibly fathom.

The plastic Fresnel lens is the next step in this modern miracle: plastics manufacturing can produce a Fresnel lens as a consumer commodity.

The circular ridges are precision crafted prismatic etchings that are the optical components of the Fresnel lens.

We live in amazing times!

Cheers,
Walter

 

[eponym]

I have one of these (Handi-Lens, by Ultra-Optix) and I"ve tried it in fresh water. The magnification I observed in water was much lower than in air. There was still some magnification, but not enough for me to consider adding the lens to my kit.

-Bryan

 

[gcbryan]

I'm just curious as to the need for additional magnification under water. Isn't water already magnifying things an additional 25%? I guess I've just never thought about needing any more magnification while underwater. Although reading my gauges is getting a little harder as I get older