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Young's Double Slit Experiment

  1. I plan on trying to recreate a version of Young's double slit experiment for a science fair. All the methods I've found for it recommend using a helium-neon laser. Would a laser pointer or some other cheaper laser work? For the slits my idea is to get a glass microscope slide, paint over it then scrape two slits off with an xacto knife.

    I jut tried using a laser pointer and a thin piece of card to split the beam, but I didn't really get an interference pattern. I think this was because the beam was to large. If nybody could recommend a good laser to use for this that would be great. Also if anybody has any suggestions regarding my idea for the slits I'd welcome them, but I think aspect of it should be fine.
  2. jcsd
  3. I have done this myself and it works. You need to persevere with making the 2 slits....make lots and pick the best.
    You also nee the laser and slits to be a reasonable distance from the screen....about 1 metre. I used a red laser. It is also possible to photograp the interference patter using a digital camera.
    Good luck...don't give up
  4. Just out of curiosity, what material are you using as a screen to capture the interference pattern with?
  5. When I did this it was in a school physics lab and the pattern was projected onto white paper taped to a wall.I used microscope slides painted with black car body paint (spray paint) and 2 fine scratches were made with a razor blade and a steel rule.
    Last edited: Feb 5, 2012
  6. Yeah, because it's visible light I'll just be using a sheet of paper. Perhaps a cardboard shade to make the pattern show up better.
  7. I am in school tomorrow.....if I can find the parts and photograph I will photograph them and post them here.
  8. Sounds good man, I'd appreciate that! :approve:
  9. Andy Resnick

    Andy Resnick 5,825
    Science Advisor
    Education Advisor

    I suspect the problem isn't the slits but the source- you can do Young's experiment using a broad-spectrum source like an incandescent bulb. The key is to increase the spatial coherence of the incident wavefront: try putting your source illumination through a pinhole prior to the slits. Making a pinhole is tricky, but a gentle pin-prick through a folded sheet of aluminum foil may be good enough.
  10. Here are some crude photos as promised. The first is the small laser held in the stand to the right. The microscope slide with 2 parallel scratches about 0.5mm apart is mounted on the piece of grey card.
    The screen is graph paper stuck to the wall about 1.5m from the slits. I took the photographs with an ipad and unfortunately the detail of the bright and dark bands does not show clearly. They are easy to see with the naked eye.
    The single slit diffraction pattern of each slit can be clearly seen and the interference fringes in the very bright central maximum are close together. (tomorrow I will try again with a digital camera that I can focus)
    I agree with Andy Resnick and before lasers were readily available I used a car headlamp bulb (which has a more or less straight filament) which behaved as a single slit before the double slit.
    The great advantage of using a laser is that it is so bright the demonstration can be shown in normal light. With the filament bulb a pretty good blackout was required to see the interference pattern

    Attached Files:

  11. sophiecentaur

    sophiecentaur 13,608
    Science Advisor
    Gold Member

    A vertical slit (parallel to the Young Slits) should do the job of collimation where it's needed and give a brighter picture than a pinhole. If your pointer is held far away, I would think the collimator would probably not even be necessary. Even the crudest laser is better than the discharge tube we had to use in the past.
  12. Did it work with a filament bulb? I would have thought that it would have to be a Xenon bulb.
  13. Mr A
    It worked well with the filament bulb. The biggest problem was very low intensity requiring a room in good blackout. The filament bulb.... a tightly wound straight coil (24W car headlamp bulb) needed to be placed a 'large' distance from the double slits. In effect the filament behaved like a single slit which is usually specified for the double slits experiment. The screen was translucent tracing paper so that the fringes could bee seen from the other side of the screen and the position of bright fringes marked with a pencil. Measurements of fringe separation could then be taken.
    The fringes were 'white light' fringes.... predominently white but with coloured edges as a result of different wavelengths (inside edges blue, outside edges red).
    It is a very reliable experiment to obtain a good estimate of the wavelength of light.
  14. Well, I was finally able to get a really good interference pattern today! I used a guitar string taped over a very thin slit in a piece of paper. I used a green laser as the source, but I think I will eventually use a red laser and a smaller distance between the slits so I can reduce the distance between slits and the wall I'm projecting it on.

    Attached Files:

  15. sophiecentaur

    sophiecentaur 13,608
    Science Advisor
    Gold Member

    You can now measure the distance and fringe separation. If you know the wavelength (it may even be marked on the laser diode or you can look at a colour picture of the spectrum and find the wavelength where 'your' green sits - near enough ), you can use the two-slit formula to find the effective spacing of the slits. Your answer should be something a fair bit less than 1mm, I imagine, but it is a good check on the method. You should get the same slit separation result when using the red source. It's nice to 'join things up'.
  16. brilliant! great to see success....check the measurements as suggested by Sophiecentaur to see if they fit the equations
  17. I measured the distance between the fringes and the distance to the screen. Using that and the thickness of the guitar string I used I calculated the incident wavelength to be about 505 nm. The actual wavelength is supposed to be 523 nm so only about 3.5% error. I'm pretty happy with that. Now I can get started on doing my own little experiment with this setup that I can take to the science fair. I'm going to get about 10 different gauges of strings, starting with one a tad smaller than I used here. Then I'll record the distance between the fringes as I increase the slit separation and see how closely it follows Young's equations. Should be fun!
  18. sophiecentaur

    sophiecentaur 13,608
    Science Advisor
    Gold Member

    Well done. That sort of thing is very satisfying! Onwards and upwards.
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