# Young's double slit gone wrong

by resurgance2001
Tags: double, slit, young
 P: 35 Hi Everyone I was trying to do a young's double slit experiment with green laser. The gap between the slits was d = 0.001m The distance to the screen was D =1.2m We counted from the center 10 bright fringes m = 10 The distance of the from the center bright fringe was y = 0.067 m Using a small angle approximation: y = (mλD)/d So λ = (yd)/mD = (0.067 x 0.001)/12 = 5.5 x 10^-6 Wavelength of laser given as about 550 nm = 5.7 x 10^-7 ouch! out by a factor of ten. Can't figure out what I did wrong. NB: This is not a homework question. I am just trying to make the experiment work, and though I am ashamed of my incompetence, I am actually the teacher! Thanks in advance for any suggestions. The bright spot in the middle was a bit big and smudged. But that was partly why we chose to count as many as 10 fringes. Even if for some reason there were an extra one or two fringes that got burnt out at the center, that would not account the factor of ten.
 Thanks P: 959 Was this done on an optical table? Do you know the coherence length of your laser? You will get the best results with a HeNe on a table which cannot vibrate ... the basement floor is better than a lab bench on the second story! From your use of a green laser I suspect you are using a laser pointer ... in that case you may also have a problem with the fixturing. I did not check the math ... I was immediately drawn to the smudged central spot.
 P: 35 Thanks. The thing was/is that the pattern of bright and dark fringes was very clear. The set up was crude. But I don't understand why I should be so neatly out by a factor of ten. I think I would like to try some more simple single slit diffraction experiments and set what happens there. I wasn't really looking for ultra precision, just a quick demonstration to show the students and hopefully to get some practice with the calculations. In fact I am beginning to wonder whether what I was actually looking at was the diffraction pattern cause by only one slit. The slits themselves were about 1/10 th of a millimeter. Thanks, I will go back and try again.
Thanks
P: 959

## Young's double slit gone wrong

You can setup the calculations in a spreadsheet both ways: single slit and double slit. Let the calculations tell you which experiment you did. :-)

If you laser beam is too small you can increase its size with a simple beam expander: http://assets.newport.com/webDocumen...expander_5.PDF

You want the beam well centered on the slits.
 Mentor P: 10,699 Yes, I suspect you're counting the single-slit maxima, not the double-slit maxima. If the width of the slits is about 1/10 the spacing between them, that would account for the numbers you're getting. Try increasing D to a few meters and doing the experiment in a dark room. That spreads out the pattern and you can distinguish more easily between the single-slit and double-slit components of the interference pattern: http://hyperphysics.phy-astr.gsu.edu...opt/dslit.html
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