optical mystery parts

[gerry maddenParticipant @gerrymadden53711]

Within the bag of loose 'bits' that came with my OMT microscope were two glass-like optical parts. I am not convinced they are actually part of the microscope or its accessories but that might be more to do with the fact that I cant see what they would do and therefore why one would use them. So here they are and I welcome your comments:-

1) Rectangular prism

This has dimensions about 7x8x20. The four long sides are black painted. The end faces are polished and you can look through the block on this axis. When you look through it the image is slightly darkened. However when you look through it slightly obliquely (in one plane only) the image becomes normal brightness but is also doubled. This puzzled me for a while and then I found that it actually appears to be comprised of two triangular prisms bonded together. The bond line is well disguised by the black paint. So clearly this bonding is doing something interesting besides making the direct image a little darker. But what ?

2) Triangular prism

This 60deg prism is about 40mm long has two polished faces and one black-painted one. It doesn't seem to do internal reflection off the black face and any image through the two polished faces is highly distorted and/or refracted into rainbow colours. Where would one use this ?

Gerry

Edited By gerry madden on 03/09/2021 13:35:23

[gerry maddenParticipant @gerrymadden53711]

[Clive HartlandParticipant @clivehartland94829]

I have seen this type of prism in a Director with an eyepiece close to the elbow. It focuses the beam nearer to te eyepiece making it a shorter design.

[Martin ConnellyParticipant @martinconnelly55370]

Have you got any polarizing filters? The rectangular one may be having this effect and could be checked with a second filter. Sunlight that has passed through a window and is on a wall is polarized by passage through the glass as is sunlight reflected off a pond surface. I imagine polarized light and filters are something that is useful with a microscope.

Martin C

[gerry maddenParticipant @gerrymadden53711]

Martin, good thought. I was about to go and find some old polarising sunglasses but then for no sensible reason put the thing in front of my computer screen instead. I didn't know these lcd screens were giving off polarised light, but surprisingly when I rotated the block the image of the screen went from clear to black every 180degrees. So you are right !

G.

[John HaineParticipant @johnhaine32865]

Iceland spar?

[BazyleParticipant @bazyle]

Posted by gerry madden on 03/09/2021 14:11:54:

I didn't know these lcd screens were giving off polarised light, .

That's how they work. The Liquid Crystal in LCD is an electronically variable polariser. Try a bit of damaged perspex in the mix. The stresses around cracks change the polarisation effects in the material and can be arranged to make pretty coloured patterns.

[Grindstone CowboyParticipant @grindstonecowboy]

Posted by Bazyle on 03/09/2021 18:30:08:

Posted by gerry madden on 03/09/2021 14:11:54:

I didn't know these lcd screens were giving off polarised light, .

That's how they work. The Liquid Crystal in LCD is an electronically variable polariser. Try a bit of damaged perspex in the mix. The stresses around cracks change the polarisation effects in the material and can be arranged to make pretty coloured patterns.

Which is why – for me at any rate – it is virtually impossible to see a smartphone screen when wearing sunglasses.

Rob

[John HaineParticipant @johnhaine32865]

You may have a Nicol Prism.

https://en.wikipedia.org/wiki/Nicol_prism

[gerry maddenParticipant @gerrymadden53711]

John, you are right ! Your first comment got the cogs turning as I remembered from geology evening classes I did many years ago about the double refraction properties of calcite.

On the same course we went on to slice up rocks into thin wafers and look at them through a polarising microscope. As soon as I saw your reference to Nicol Prisms it all came flooding back.

And picking up on Bazyles point, whilst watching the news last night I found could peer through the prism and rotate it. At a certain point the telly screen went black as if had switched it off, but everything else in the room was normal. I then found that I could hold a clear plastic disc at arms length to restore visibility of small sections of the telly's screen to its normal appearance, albeit with some fringes due to the stress in the plastic.

All fascinating stuff indeed. But the question remains, is there any use for a Nicol Prism in the context of a toolmakers microscope ?

Gerry

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