Prism Play

Forum to discuss all areas of lighthouse technology such as optics, fuels, fog signals, radiobeacons, daymarks, construction, etc.

Postby island » Mon Oct 01, 2012 7:19 am

Oh horrors! Technical stuff!!

There are numerous written descriptions of how prisms work in a Fresnel lens. For the fun of it I created the following using a prism with direct sunlight to demonstrate how a prism impacts light as occurs with a Fresnel. The brightest straight beam of light is the refracted light from the prism, to the left in the first image and to the top in the second. The source light was early morning sunlight that entered the prism on the side directly opposite from the black shadow. The transmitted light was projected on a piece of white cardboard. The fuzzy appearance of the bright beam in the bottom photo was caused by imperfections in the prism glass. The secondary weak beam was the result of not being able to directly focus a sufficiently narrow beam of sunlight.
Prism 001.JPG
Prism 001.JPG (79.97 KiB) Viewed 5142 times
Prism 004.JPG
Prism 004.JPG (74.71 KiB) Viewed 5142 times
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Postby Kevin vk2ce » Mon Oct 01, 2012 8:35 am

Afew years ago I developed Dilopia which simply put is double vision. It's caused by infection of one of the cranial nerves, 5th I think, which causes the eye to move from left to right. There are 5th cranial nerves on both sides of the heard and along with anoth 5 or 6 nerves they go through a very small cavity in the ear. If both nerves are OK then the eyes move in sync but if one is infected the eyes don't move in sync and you end up with double vision.
The Optometrist I went to prescribed a Fresnel lens which was a piece of ribbed plastic which he stuck onto the lens of my prescription glasses for the affected eye to look through until normal vision was restored. Miraculously it worked and fortunately the damaged nerve repaired itself and vision returned to normal after around 3 months. Fresnel lens principles are also used in with plastic magnifying sheets for making it easier to read books with small print.
He was indeed a clever chap.
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