Non-Coherent Light Diffraction through Double Slit

[tapanther]

Hi, I'm trying to perform a double slit diffraction experiment without using a laser, but instead using a super-bright, white LED. This LED has been placed inside a sealed box with a small pinhole opening (~1/32 inch diameter). This however introduces the problem that the light is incoherent, and rather than seeing a diffraction pattern I see two slits of light projected across the room. I have tried distances ranging from 5 feet to well over 50 feet (the latter using a camera as an integrator to see any pattern that may be invisible to the naked eye). In all cases, I see only 2 slits, or two almost superimposed slits (like a wide single slit with a dark patch running down the center, not diffraction just two lines of light that are almost on top of each other). I thought about using a polarizer and a red filter to make the light as close to coherent as possible, but this did not help. The diffraction pattern appears when using a laser however, as expected. I am wondering what could be the problem. I've tried varying the wall-slits and slits-light distances, basically all possible combinations. I've tried using only a red filter (very good red filter, only allows a small bandwidth through), using only a polarizer (to make all the light oscillate in the same direction), both red filter and circular polarizer, but nothing has worked. I am concerned that perhaps the material my slits are in is too thick for the LED light, since its 1/8 inch thick, and most experiments are done on cardstock or a thin opaque film. Does anyone have any suggestions that may help create a diffraction pattern? Using a laser is not an option, as that defeats the entire purpose, and I know that it can be done using non-coherent light, as that is how Young and Fresnel did it.

Thank You!

 

[Andy Resnick]

The double slit experiment requires spatial coherence, that's true. A 1mm diameter sized visible source placed 1 cm from the apertures gives a coherence area of about 2*10^-7 cm^2- in other words, the slits need to be about 0.005 mm apart to generate interference fringes. Making the source smaller (smaller hole), or moving the LED further away from the slits will result in a larger coherence area, allowing the slit spacing to be larger.

Also, in order to make the fringes sharp, monochromatic light is generally used. Do you at least see rainbows with the LED?

Early experimenters used a filtered arc source (say a Hg arc, a Na lamp, or even a Carbon arc), which is substantially brighter than a superluminescent diode.

Does this help?

 

[tapanther]

Unfortunately I have tried using the light source well over 5 feet away from the slits and see nothing (note I am using a very high quality digital camera to integrate over ~30-90s exposures). Even with the light this far away, the camera clearly records two single slits. I've used monochromatic light (or at least very close to it, using a red filter), but it didn't help. I did not see rainbows with the white LED. I can attempt to use a smaller hole, the camera has very little trouble picking up really faint light since there's no extraneous light sources (I was working in a darkroom).

I also do not have access to an arc lamp, the physics dept here is closed for the weekend, and not being a phys major myself I don't have access.

Thank you though, this gives me a few ideas to try out. I'd still appreciate any other advice you may have.

 

[Andy Resnick]

hmmm... let's start from the beginning. Can you provide information about the LED (manufacturer and model number)?