# I Strange Optical Phenomenon (diffraction or something else?) (solved)

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1. May 2, 2016

### Staff: Mentor

Sorry, forgot about that thread.

If we would see a grid diffraction pattern, the pixels could not be visible at all. You cannot have interference between adjacent pixels if they are resolved in the image.
You see light if two conditions are satisfied: (a) the direction is right for light coming from the pixel (it is a camera/eye observing the pattern) and (b) the angle is right for positive interference.

The last picture is not from an extended grating, it looks like a double-slit or maybe triple-slit pattern. Otherwise the maxima would be much smaller and sharper.

I tested it with some more monitors. Some show nice patterns with variable angles, some do not show a pattern at all.

2. May 3, 2016

### Tazerfish

The reflection of the Laser does have extremely pronounced maxima.(take another look at the pictures)
It couldn't have been produced by a single slit or double slit.
I trust the observations that much.
I think we agree on that right ?
But that seems like a big problem:
I finally get why that is now, although it took a while.
Thanks for beeing insistent after i falsely declared this problem completely solved.

I ran the numbers for the grid spacing you would need to have for this pattern.
Since i still think it looks like a grid.
$n \lambda = \Delta s$ for constructive interference with a grid
$\Delta s = sin(\alpha) l$ l is the grid spacing
alpha the angle at wich the light exits

I first thought we would have to consider the optical density of the material in wich this happens.
(The angle of outgoing light in the material is different due to the shift in wavelength and then the angle changes when leaving the material due to refraction.)
Now i think both changes cancel each other out.

After solving for l and plugging in approximate values for lambda and sin (alpha) i get the VERY rough estimate of
$l=2*10^(-5) m$
The reality could easily be twice or half that.
PS:What is actually the correct way to write such an extreme uncertanty? usually you do it with plus/minus some value or percentage but that makes no sense here.

The previous result is far from the real $l=\frac{1.2m}{1920*3}=2*10^(-4)$

It is off by a whole order of magnitude !!!!
So my initial assumption is completely off the table.
And to make it worse it is not small enough to make a grid between the pixels....
I thought maybe the dark lines between the subpixels consist of multiple lines.
For example, one in the middle grounding and two on the sides which each control one subpixel voltage.
Somewhat like the thin wires in this
But i don't think that is true.

NONETHELESS IT LOOKS LIKE A GRID.
I can see up to about the sixth maximum.
That has been confirmed by an experiment with a laser pointer.(disclaimer i did actually shine the laser in my face with a one of the "other maxima"
but my eyes are all good the laser is only p<5mW)

BTW I am talking about the screen from the beginning.The one which had diagonal pixels.
Do you know of any structure that is somewhere around ten times smaller than the subpixels (but parallel to them)?

3. May 3, 2016

### Staff: Mentor

The reflection of a laser can be different thing, the maxima are on a screen.
I don't know. The polarization filter has to have even smaller structures to work.

4. May 4, 2016

### Tazerfish

I think they are the same ...
Just imagine you put one eye where one of the secondary maxima is projected.
You would then see a bright dot on the screen,where the laser hits the screen ALTHOUGH the beam is not directly reflected into your eye.
That would be true for all the positions where some not-primary maximum hits your eye.
So when shining a really big laser at the screen you would only see the points which would project some maxima into your eye.
The patten which emerges form this is very similar to the projection you get when only hitting one part of the screen.

It is similar to the fact that a single raindrop will project a whole rainbow around it, but very many raindrops all over the sky project white light back.
But a camera or human can see a rainbow in the sky similar to the projection of one drop.

I don't know whether i explained that very well.
Thats really cool btw... and it might take a pen and paper to really get it if you never thought about it that way before.

An interesting observation:
In the reflection patterns there is usually one very bright maximum (the reflection off the glass surface)
And then there is a grad pattern for most screens.
The interesting thing is that the center maximum of the grid reflection is slightly offset compared to the glass reflection.
That seems to confirm that the laser enters the screen get reflected off the back polarizer and exits again(after interactin with something).
That extra distance in the screen produces the offset.

5. May 4, 2016

### Staff: Mentor

An eye has a lens and its "screen" is behind its optics. An eye without a lens would see a completely different world (most notably: extremely blurry).
A camera/eye can. A screen cannot (unless you add optics).

6. May 4, 2016

### Tazerfish

No you dont get what i am trying to say.
This is a little tricky and i am not particularly good at explaining it.

But the PROJECTION of ONE ELEMENT.
Is very similar to the PICTURE of MANY ELEMENTS.

You can think this through yourself you you can just trust me.
If you had a "bigger" monochromatic light source
and shone that on the screen you would see a pattern on the screen almost exactly like the pattern in the reflection of the small laser.
The light source would have to still be parallel but covering a significant potion of the screen.
I tried a lens to expland the laser beam but it wasnt really good enough.The lens(magnifying glass) was too weak.

Yes i knew what i was talking about.
The same with this. I am not confusing this.

I am very certain that this
The PROJECTION of ONE ELEMENT.
Is very similar to the PICTURE of MANY ELEMENTS.

and
If you had a "bigger" monochromatic light source
and shone that on the screen you would see a pattern almost exactly like the pattern in the reflection of the laser.

are true.

Last edited: May 4, 2016
7. Sep 9, 2017

### Gustavo Kleis

I had different results using a plasma tv and iphone flash light. can see, an interference pattern, only sees the color red, two lines forming an X horizontal, in the middle there is no light, even turning the flash light, the angle remains the same. I remembered that, I searched the internet but nothing ... TEST on a Plasma TV, crazy results

Last edited: Sep 9, 2017
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