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# Rotate prism 45°, image does 90° - why?

#### Presentteck

##### Member
I found this prism a few years back and was amused with its rotational effects. When rotated around the viewing angle, the image rotates twice as fast as you move it.
I couldn't find any explanation, possibly because I don't know the correct search term to find it.

I took the prism out of its housing to find out why today.
The base side is mirrored, and it's an isosceles triangle. 62.046 base angles and 55.908 at the peak.
I did some math, then I just shined a laser close enough to the side to see, and marked paths. Way easier.
Since theirs a mirror the traced paths shown make total sense on paper. But I can't wrap my head around the rotating 2x movement effect with the 2D representation.
Does anyone know what this is called or how it works?

Here's a link to a bunch of images of the effect.
Also have full sketch of prism, including light paths.

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##### Well-known member
I found this prism a few years back and was amused with its rotational effects. When rotated around the viewing angle, the image rotates twice as fast as you move it.
I couldn't find any explanation, possibly because I don't know the correct search term to find it.

I took the prism out of its housing to find out why today.
The base side is mirrored, and it's an isosceles triangle. 62.046 base angles and 55.908 at the peak.
I did some math, then I just shined a laser close enough to the side to see, and marked paths. Way easier.
Since theirs a mirror the traced paths shown make total sense on paper. But I can't wrap my head around the rotating 2x movement effect with the 2D representation.
Does anyone know what this is called or how it works?

Here's a link to a bunch of images of the effect.
Also have full sketch of prism, including light paths.

I'm sorry I cannot open the image as my current connection blocks facebook. But this might be you are looking? Link

Edit: Here is also a nice pic:

Edit 2: Now I saw your pics - yes it acts as a Dove prism, look at the pic under c above - this is how it refract and reflects. Mirrored coating helps a lot and the top corner might be cut off in some versions of these prisms.

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#### BobMc

##### Well-known member
I'm sorry I cannot open the image as my current connection blocks facebook. But this might be you are looking? Link

Edit: Here is also a nice pic:

##### Well-known member
Thanks.

BTW: To add - when thinking about the drawing of light paths - it seems to me that there are captured aditional internal reflections on the drawing, aren't they? I mean that the light travels as captured on figure c, but it might look from outside like in the drawing. That would explain significant diference.

Another self explanatory pic:

(source linked to the image as well)

#### BobMc

##### Well-known member
I'm still trying to wrap my head around the light parhs. Going to have to reread. Very interesting through .

##### Well-known member
I'm still trying to wrap my head around the light parhs. Going to have to reread. Very interesting through .
It is quite simple - just look at the latest image I posted. See original object - the light from it just enters the prism, refracts, than it reflects from mirrored side (with all the transformations like in the mirror) and refracts again to exit the prism in the direction it entered it. I would draw the R letter on the mirror side (or imagine it) and it is clear.

Edit: The mirror side is on the bottom on left pic and right on the right pic.

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#### Presentteck

##### Member
Thanks for the info. Moving in the right direction!
So I went with dovetail keyword search, starting to make some sense of the translative effects... vaguely

However, my prism is not a right angle prism which means it's not a true dovetail, and has more going on in it.
The light path is more complex, refracting many times.
and it appears that the exit side comes out a slightly larger image than the entry side. I tried to line up the beam parallel with the lines I drew, but this could be off. I should retest with a static, not in hand, pointer...

If I'm not wrong my prism acts as a translator and an image size modifier.

Check out this video on dovetail prisms I found.

##### Well-known member
If you know material you could find refraction index of it and it might help you out to get true angles of refracted light inside. I still believe the optics happenig is quite simple.

It would be nice to put some laser through on longer exposure to see the beam on photo.

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#### Presentteck

##### Member
Ta-Da!

the more i look at this thing the more i think its an image corrector for a telescope, but just a guess.
after hitting the collector mirror it would need mirrored again to straighten it out, rotate to any angle, and this would slightly enlarge the image similar to a beam enlarger... but not enough to notice it on a single laser beam

uploaded images of housing, same album

cast number 8147, M937©, 814796. back plate: 1123355. bottom plate: 1010524, and a stamp seen in last two images of album

##### Well-known member
Ta-Da!

the more i look at this thing the more i think its an image corrector for a telescope, but just a guess.
after hitting the collector mirror it would need mirrored again to straighten it out, rotate to any angle, and this would slightly enlarge the image similar to a beam enlarger... but not enough to notice it on a single laser beam

uploaded images of housing, same album