I recently posted the idea of a "magnifying spherical shell" where (parallel) rays of light from any direction focus on a point in the centre. I thought that idea could be modified to make a one-way-ish window.

Start with an array of hemispherical magnifying shells on a sheet of glass. Each hemispherical shell having a black dot in the centre. Looking at a hemispherical shell from any angle will look black (since the black dot is magnified).

See cross-section diagram.

View from the front
Looking at the front of the array will look black because of each magnified black dot.

Light will also be transmitted through the window. This will be out of focus.

So each hemispherical shell will be pixel of black plus the out of focus light.

View from the back
Looking at the back of window, you will see an array of tiny un-magnified black dots. You will also see all (non-parallel) light which has made its way through the lenses. All the parallel light (that is light from infinity) gets absorbed by the black dot.

Each hemispherical shell will transmit (mainly out of focus) light.

In summary
The window will look mainly black from one side so you can't really see through it. The other side will allow you to see through (albeit pixellated).

Might be useful as a shower screen, or anywhere else an imperfect one-way-ish window could be used.
-- xaviergisz, Sep 22 2021

magnifying spherical shell
[xaviergisz, Sep 22 2021]

cross-section diagram https://i.imgur.com/KwEfKOm.png
With rays of light shown [xaviergisz, Sep 22 2021]

That's so crazy it just might work! Do you know any glass- workers that could mock a section up for you? Probably a mold would be the best way to make it.
-- neutrinos_shadow, Sep 22 2021


//View from the front Looking at the front of the array will look black because of each magnified black dot. Light will also be transmitted through the window. This will be out of focus. So each hemispherical shell will be pixel of black plus the out of focus light.// - I think this is wrong. You will see a magnified black dot but nothing else - no other light transmitted through the window (except between the blobs of there’s any room between them. From the back, the blobs will act like cat’s eyes or retroreflectors.
-- hippo, Sep 22 2021


The idea requires high refractive index material so the best person to make it might be a gem cutter/polisher. Cubic zirconia gems are quite cheap.

But before doing that I would probably get an optics expert to do a simulation. It is possible that such a thick lens wouldn't work as well as the 'lens makers equation' suggests.
-- xaviergisz, Sep 22 2021


If you draw a ray-trace then it is meant to be symmetrical right? So a ray that goes through the lens and hits a black dot works in either direction. So if the side A viewer looks and sees only black dots magnified, then the side B viewer will not see anything from side A because the tiny black dots are absorbing all of the light from side A by definition. i.e. won't work.
-- pocmloc, Sep 23 2021


[pocmloc] I think that's right - from one side you'll see the black dots magnified, from the other side you'll see a retroreflective surface
-- hippo, Sep 23 2021


[scad mientist] has pointed out in the "magnifying spherical shell" idea that I've stuffed up the calculations. The idea is only (possibly) salvageable by using gradient refractive index materials.
-- xaviergisz, Sep 23 2021


(+) for the thought process.
-- 2 fries shy of a happy meal, Sep 25 2021



random, halfbakery