Evidencing interference pattern using Young's slits and sunlight - help required

[David2001]

I am an artist with some knowledge of physics and I am trying to build a pinhole camera to expose photographic film using sunlight that passes through a filter, pinholes and Young's slits. I would like to photograph an interference pattern using the sun as the subject.

My prototype camera isn't working and I wonder if anyone could help?
I positioned a digital camera at the film plane and took long exposures but the images I got are without fringes - eg a blurry garden shed without any kind of ghosting or evidence of fringes. The final quality of the image isn't important - blurry or not - as long as there are fringes of some description.

I wonder if I am capturing only one of the fringes and missing others appearing either side?
My next thought was that perhaps the light is not coherent enough, and that this was a major issue.

The current prototype camera consists of:

| red filter | pinhole | pinhole | Young's slits | film plane |

I understand that sunlight is not the same as laser light, but I'm using a red photographic filter to try to make the light monochromatic. I am hoping that light passing through two pinholes will increase the degree to which it is collimated (but would sunlight be collimated enough at this distance for this to work without the pinholes? Would one pinhole positioned in front of the Young's slits be sufficient?)

I guess there is an issue with coherence - is it possible to make the sunlight coherent enough to allow this to work? Could I filter it somehow?

Would building a Mitchelson interferometer arrangement within the camera be of any use to me?

The pinholes are made using a needle to drill aluminium from a drinks can.
The double slits are made using finely cut and taped pieces of an aluminium drinks can - I can achieve fringes using a laser pointer shone directly at the slits (out of the camera).

The initial pinholes are about 50mm apart. The slits are about 250mm behind the last pinhole, and the film plane is about 250mm behind the slits.

I'm about to build a second better prototype camera with 140mm separation beween the slits and the film plane, but I'm not sure how far I should separate the initial pinholes, whether I should remove one, or how far the final pinhole need to be from the slits.

Any help would be very greatly appreciated, many thanks in advance.

 

[Cthugha]

Not sure that it will help much, but I will try.

Spectral filtering of your light was a good starting point. Although a double slit measures spatial coherence and not temporal coherence you would get a superposition of many interference patterns for different wavelengths when having white light. I do not know how broad your filter is spectrally, but it should suffice.

It sounds rather like you need to ensure spatial coherence. In principle you can trace a line from every point of your light source to the slits. These lines will of course have slightly different length. The difference between the longest and shortest line you can draw that way will determine whether you will see an interference pattern or not. If the distance is larger than the typical length scale over which the coherence of your light vanishes, you will not see an interference pattern.

Therefore there are two basic things you can do to increase coherence:

1) Use smaller pinholes. The effective size of your light source (which is the pinhole size) will go down which in turn increases spatial coherence at the sit positions.

2) Place the slits further away from the last pinhole. Although this seems counterintuitive, it will reduce the spread of emission angles you collect at the slits and therefore also the difference between the "longest line" and the "shortest line" between your light source and the slits.

Both of these possibilities will of course reduce the intensity of light you will get, so it might pose a problem.

You can also decrease the distance between the slits to increase spatial coherence at the slit position, but this does not help that much.

 

[dipole]

You may also need to decrease the width of the slits themselves, how did you make them how wide is each slit?

 

[David2001]

Thanks for your help Cthuga and dipole.

I made the slits by cutting a thin strip out of a small square of aluminium (from a drinks can) and then I positioned a thinner vertical strip centrally along the hole and used tape top and bottom to result in two slits. The whole thing is mounted in a slide mount. I'm attaching a pic with a ruler to show scale. The diagonal lines are scored lines to find the centre.

Thanks again for your help.