Problem with the two slit experiment, Observing later

Hi C2,

So if I understand you correctly, you were intentionally changing the distance
and viewing angle of the camera relative to the source....you should have
described what you were showing and attempting to do, it would have
put things into proper "perspective". Your lack of information left us/me
with the idea that the camera was stationary and that the image was moving
on its own over time.

LL

This post has been edited by Laserlight on Apr 22 2007, 03:45 PM

What's not to like about that?
Best wishes,
-C2.

Hi Laserlight, Confused2, yquantum, Jal, Neil Farbstein et al,

What I mean is that there are two kinds of coherence... spatial and temporal. In most cases we are dealing with both with lasers. I have indicated in the past that temporal coherence is a strong organizing influence but pure spatial coherence could also occur say for instance with a quantum dot source of individual photons emitted at random times. If spatial source phase was maintained within a fraction of a wavelength then photons emitted at any subsequent time from this very compact source could lead to identical interference fringing (if it was possible to observe) throughout the surrounding environment. This could lead to creating a hologram built up using individual photons, provided they were strongly spatially coherent by originating from the exact same point in space and had the same source phase on emission.

Lasers virtually "regiment" photons to be temporally coherent even though the photons do not share the same individual sources but they are synchronized by virtue of "stimulated emission".

Pure monochromatic sources can also "develop partial coherence" due to the self organizing influence of co-moving photons. We must still remember this was the first way in which the original fuzzy holograms were produced in the dim days before LASERS.

Cheers

Hi Good Elf,
Is there any evidence that would help to decide the standing wave issue? I've got pictires of DSE patterns taken at different distances and angles - they all look to be exactly as I would expect on the basis of path length difference and very difficult to account for in any other way. Would there be any point in my posting them?
Best wishes,
-C2.

Hi C2,

Can you do an experiment with your camera setup? If so, it should be done like
this:

1. the camera detector should be directly perpendicular and centered between the
holes or slits with a starting reference point of 10- 20 cm from the apertures.

2. mount a ruler or some accurate measurement device along the length of travel
of the camera "carriage", and mark uniform spacing increments along the length of
travel to use as alignment marks where you will take a picture at each mark.

3. take at least 6 pictures at calibrated distances of at least 2 cm between each
calibration mark.

4. Post the pictures and provide your observations, opinions.

Is this asking too much?

Regards,
LL

Of course it isn't asking too much. I will do my best.

-C2.

20:-

18:-

16:-

14:-

12:-

10:-


Apologies to anyone on narrow band.
More to come

Now individual pinholes with low then high gain

These are at about 16 cm frm detector









This post has been edited by Confused2 on Apr 22 2007, 10:45 PM

C2,

Nice job! I understand the first set of pics, but don't know what to make of
the second set, except that there are no observable interference fringes/bands.
The high gain pics are showing some ghost image artifacts, perhaps standing
waves or in phase components of the Airy ring.

The first set:

As expected, the interference fringes and gaps increase/spread with
increasing distance from the source and resolution degrades.
The patterns seem to be following the ISL. There appears to be an increasing
image projection pattern that tracks with increasing distance.

Your thoughts, comments, and anyone else can comment also.
LL

This post has been edited by Laserlight on Apr 22 2007, 11:39 PM

Hi LL,

The last four pictures were intended to show the diffraction pattern of each slit. My pinholes don't seem to be very good hence my Airey rings are a bit hairy fairy.

My camera thing seems to ignore things it doesn't think are important unless I get the gain just right. Here is a single pinhole taken at about 14 cm after playing about with the gain



Hopefully it shows that the single hole diffraction pattern is much more extensive than was clear in the last four pictures.

In my corner I'm pretty keen on the view that the DSE effect is no more (!) than the superposition of one diffraction pattern on top of another .. it can't add anything that wasn't there before. My camera has a bit of a life of it's own which hasn't really helped to prove this. I must admit I've only just worked out how to block one slit at a time. Ideally I'd prepare yet another set of pictures which show one slit, then the other and then both.

FWIW .. here is a 3 pinhole picture



Best wishes,
-C2.

Hi Confused2, Laserlight, yquantum, Jal, Neil Farbstein et al,

Just a question, are the two blobs with internal fringes produced by two sets of pinholes (ie: 4 pinholes?) each set separated by some distance?

Or is this one pattern produced by only one set of pinholes and they are interfering over a relatively very great separation distance?

In the three pinhole picture... in your post above... are these pinholes arranged in an approximate triangular pattern?

Is this image only one quarter of of a single diffraction airy disk, is that 2 irregular airy disks I see there?


Previously you said...

Is the "bog roll" still producing that outcome above or something like it? Are you still using that blue tack with two tiny pinholes in it? What really interests me is are you reducing the various higher modes yet by using a "spatial filter" on your laser and are you using "diverged laser illumination" in the Fresnel Zone?

Ideally you should have the laser initially a "large" distance from the slits and the source should be "spatially filtered" with another largish pinhole that "crops" the "edges" of the laser lens.. This should improve things.

Just trying to understand why it is currently so "messy"? Otherwise it is interesting. What are you intending to discover here?

Have you got an explanation for the unusual colors... ?? Shouldn't they all be the same color (ca. 660 nm red? or is this a green laser?) if you are using only one laser source? Is this an effect due to the patterned color filter behind the lens but over the sensor... Are you focussed on these tiny lenticular lenses??

Otherwise .... well done... definitely "food for thought".

Cheers

This post has been edited by Good Elf on Apr 23 2007, 11:26 AM

Hi Good Elf,

The active area of the 'detector' is about 2.5mm by 2.0 mm. There are no lenses and I have removed the IR filter .. the light goes straight onto the chip. The colours haven't been 'right' since I removed the IR filter. I'm not sure the effect is related to the removal of the filter .. my feeling is that it's just leakage between cells which is wiping out the colour .. it can still be seen in low amplitude areas. Laser is always 650nm red.

Until the last '3 pinhole' picture all pictures have been produced using a piece of black card with two pinholes poked through .. this is then stuck with bluetack to the front of the laser pointer. The source is the laser 'as is' .. the 'spot' is about 4mm in diameter at the exit of the device and it diverges to about 8mm at a distance of about 3m . Since my pinholes are about 1mm apart the 'spot' covers both pinholes and gives a good DSE effect .. I didn't expect this to be true but since it works and keeps everything simple I've just accepted this bit of strangeness.

Since the laser beam hardly diverges over the region of interest and we know the dimensions of the sensor we can effectively measure the pinhole separation by the distance between the bright round bits in most of the pictires. In the first picture you posted above the separation is seen to be about 1.25mm.

Most of my pictures are taken with the slits about 10cm from the detector.

The thre pinholes are in a triangular pattern .. sorry, I should have made that clear.

Airy discs .. the source is over to the right .. the discs are irregular but repeat .. I see maybe 3 discs.

"Is the "bog roll" still producing that outcome above or something like it?
With or without the bogroll makes no difference it just keeps light from my PC screen away from the sensor. I regret there is no difference with or without the bogroll, poking fingures down the side etc.Nothing makes any discernable difference unless it is in the direct path between the laser and the detector.
".. are you reducing the various higher modes yet by using a "spatial filter" on your laser and are you using "diverged laser illumination" in the Fresnel Zone?"
God only knows! All is as described.
"What are you intending to discover here?"
If we use DSE equation ( http://schools.matter.org.uk/Content/Inter...ce/formula.html ) and assume my slits are about 1mm apart , wavelength 650nm and 10cm to the 'screen', the screen being about 2.5mm across then we'd expect to fit about 40 bright bits across the screen.

Looking at (10cm to screen)
http://www.modellines.net/test/Picture%2039.jpg we see a spacing equivalent to about 40 fringes across the screen and I guesstimate there are actually about 46.
And here (20cm)
http://www.modellines.net/test/Picture%2034.jpg
I guestimate about 21 fringes.
I'd say this is entirely within the limits of how accurately I actually measured these things.
For accuracy we have the Teachspin results .. I'm only looking at "What's going on". We are seeing effects that are not obvious in a 'proper' setup.
In many of the pictures we see the DSE fringes are unevenly spaced. My explanation is..
If we imagine the larger scale diffraction pattern as 'background' then the 'ray' from the other slit has to cancel the (variable) phase of the 'background'. I agree this is something that is not taken into account in my simple DSE equation .. we have much better equations which we don't really need here .. as long as we understand the principle.
"Is this an effect due to the patterned color filter behind the lens but over the sensor... Are you focussed on these tiny lenticular lenses??"
No lens and no filter. Sorry, no evidence of any cavity effect.
Best wishes,
-C2.

Sorry a bit hurried .. do come back on anything I've missed.

Hi Confused2,

Thanks for that. It clears up a lot of unknowns.

Here is a page that indicates how a color camera works and how the color filter sits over the CCD sensors on the chip surface.
http://www.shortcourses.com/choosing/how/03.htm
However this is what it generally looks like...

If you have a color camera and you only removed the large optical lens then this three layer "sandwich" remains composed of a clear lenticular bead lens array (focusing the light slightly into the center of the patch), transparent color patch array, then underlying all that the CCD pixel sensors. The color of each pixel in the image is "interpolated" as shown in the final illustration using an algorithm using the 8 nearest color sensors and itself using their basic response to light.

Naturally each CCD patch is not naturally color sensitive only light sensitive until it has this tiny color grid laid over it. An onboard processor chip jiggles the individual values around. As indicated in the page referenced, they usually have the "Bayer Pattern" which has twice as many green patches as there are red or blue patches and this means it is twice as sensitive to green as it is to other colors (including red). In theory these three particular colors, If you have a red laser, then the green and blue filters should block most of that pure red light in about 7/9ths of all the pixels. Because your pictures have a pattern it is possible this influence is causing an "artifact". I am pretty sure that the strong laser illumination penetrates all the filters maybe even having a non-linear effect, and I am expecting a banding effect due to this phenomenon because of this filter. This may be second order... I dunno what it might do here. Have you any comments to make about it being "Johnny on the spot"?

Cheers

This post has been edited by Good Elf on Apr 23 2007, 04:18 PM

Hi Good Elf,

I'm under the impression that cheap CMOS cameras like this have very crude colour filters which are made by etching the SiO2 down to different thicknesses over the active bits to get 3 colours. Obviously they aim for RGB or similar but my guess is that it's more like high, medium and low frequency.

Initially I tried to make my pinholes as small as possible but once you find anything works regardless of size the temptation is to pop 'em out rather less carefully.

As far as I can tell the thing is telling the truth .. there are no oddities when rotating, moving in and out and sliding across. I have put the lens back and it still works as a camere.

The pictures I've posted so far have been jpegged .. the early ones were more compressed but the later ones are 400K after jpeg -- as a BMP they take 900k so I doubt if there is any detectable difference as a result of compression.

I hope this answers the questions.
Best wishes,
C2.

Hi C2,

Excellent work and descriptions for your home brew experimental setup!

Are you up for trying to use slits rather than pin holes?

LL