I tried, and failed, to do the double-slit experiment. I thought I had a good experiment setup, but obviously I was wrong. Can anyone offer some insight in what I might have done wrong? Most of this was done using some vague memory of experiments way back in High School, so there might be a better approach.
I did search through past questions. This question was the closest to what I am shooting for, but there are significant differences and did not enlighten my particular situation.
Setup:
I used a variety of laser pointers as the light source, both red and green. One laser pointer was home built using a red laser diode (200-250 mW) from a DVD burner going through an adjustable lens.
The slits were made by depositing soot from a candle onto a microscope slide, then "cutting" slits in the soot using razor blades. Two blades were held together and the slits "cut" at the same time with a single swipe of the razor blades. Initially I used old fashioned double-edged razor blades but the blades were too thin and only a single slit was made in the soot. I switched to X-Acto blades, which made two slits approximately 0.5 to 0.8 mm apart. I didn't measure this, I just eyeballed it. I did both a single slit and a double slit.
I varied the distance from the laser to the slit, from inches to 20 feet. The distance from the slide to the wall was varied, inches to 8 feet.
Results:
I never clearly saw an interference pattern. I saw things that were probably some kind of interference pattern, but there was no consistency. One experiment showed a pattern of lines that was about 2 cm apart, while just changing the "quality" of the slit changed the pattern to about 0.25 cm apart (not changing the laser/slit/wall distances)– this just seemed wrong to me and implied some fundamental experiment setup problem.
I also expected a difference between the single and double slit, but that was not the case. The variation of patterns seen for a single slit was similar to what I saw with the double slit.
When the laser-slit distance was 20 feet, and the slit-wall distance was inches then I could clearly see an impression of the slits on the wall with no hint of an interference pattern.
Other experiment notes:
The soot was surprisingly difficult to put slit into. If the soot was made thick enough to block all light then the edges of the slit were ragged and the width of each slit was not consistent. If the soot was thinner then the slit was smoother and consistent, but there was some light leakage in the areas that should have been solid black.
Questions:
Is the "soot on a microscope slide" a valid method for making the slits?
Is the laser pointer with adjustable lens an appropriate light source?
Should I expect to see a diffraction pattern with only the double slit, and not with a single slit?
Did you see any other improvements I could make to this experiment?
Best Answer
It sounds like the laser beam was too narrow to go through both slits.
This video shows the use of hair instead of two slits. That gives you a more stable interference pattern.
I had the same problem with soot that you did, so I used a sheet of aluminum foil with two cuts made with a knife, but that still doesn't allow the nice calculation shown in that video.