My Young's slits experiment doesn’t work. I mean I tried to see wave effect of light but I didn’t see any thing expect particles! I’ve sent you a pic of my laser and a short video of the experiment. Is there any problem you think?

Click here for the video

enter image description here

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    $\begingroup$ The video link takes me to a page of arabic that I couldn't understand. However note that in the Young's slits experiment both the slit width and the slit seperation need to be around the wavelength of the light i.e. around a micron. The slits shown in your picture are much too big and much, much too widely separated to produce an interference pattern. $\endgroup$ Jul 7, 2016 at 15:58
  • $\begingroup$ Related question, how do I perform double slit experiment at home ?physics.stackexchange.com/q/54052 also youtube video youtu.be/kKdaRJ3vAmA $\endgroup$
    – user108787
    Jul 7, 2016 at 16:09
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    $\begingroup$ The slit width and spacing need to be small, but they don't need to be that small. Consider physics.stackexchange.com/questions/155216/… for an example with slits much larger than the wavelength. However, the bigger the spacing the longer the projection distance you need to get a clear pattern. $\endgroup$ Jul 7, 2016 at 16:10
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    $\begingroup$ john rennie . In one of the physic books I read and I saw this with identical slit and sorry about that page . it is not arabi it is FARSI . $\endgroup$
    – mahsa
    Jul 7, 2016 at 16:18
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    $\begingroup$ See this question and my answer to it. Can you reproduce that experiment? It will at least tell you that you have a laser (and not a red LED). Next shine your laser at a single hair in a dark room - you should see a line of dots appear on a screen behind it. Finally - you really need thin AND STRAIGHT slits otherwise the interference pattern will wash out. $\endgroup$
    – Floris
    Jul 7, 2016 at 17:07

2 Answers 2


The angle between maxima in the double-slit pattern is $$ \theta \approx \frac\lambda d $$ for wavelength $\lambda$ and slit separation $d$. I wild-guess that the slits in your photograph are about 5 cm apart, so your diffraction peaks should be separated by $$ \frac\lambda d = \frac{\rm 500\,nm}{\rm50\,mm} = 10^{-5}\rm\,radian $$ which is too small for you to resolve the peaks even if you were illuminating both slits at once (which you're not).

So, your slits are too far apart. Try splitting your beam into two parts by straightening out a staple from a stapler and shining the laser on the staple; that works pretty well. You'll need to jerry-rig some mount for the staple and laser, since the diffraction pattern isn't obvious unless you can get it on the other side of the room.

Here's a quick setup I threw together. I stood a staple up on a pencil eraser and used it to split a laser beam:

staple standing in eraser laser being split by staple

The spot is on a wall about four meters away:

laser spot viewed from across room

From up close, you can see there are about three or four maxima per fingers'-width: diffraction pattern up close

A hair is narrower than a staple, so you'd more distance between maxima if you split the beam with a hair.

  • $\begingroup$ so what about the slit in the pic of laser ? are them bigg? no I conducted it with a tiny tiny tiny hole and laser and red light and perpule lite but I didn't get the result . $\endgroup$
    – mahsa
    Jul 7, 2016 at 16:47
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    $\begingroup$ The slits in the cardboard in your photo are too far apart. Try a staple, or a thin wire, or a strand of hair, to split the beam. $\endgroup$
    – rob
    Jul 7, 2016 at 16:49
  • $\begingroup$ no look at this pic . it dosnt work i.imgsafe.org/e8a86ba550.jpg $\endgroup$
    – mahsa
    Jul 7, 2016 at 17:00
  • $\begingroup$ Ive passed light from that tiny hole and have conducted it by this light again . there is not any advantage =i.imgsafe.org/e8b5a69212.jpg $\endgroup$
    – mahsa
    Jul 7, 2016 at 17:03
  • $\begingroup$ @rob I like the legend of "GILGAMESH" so much, but I have a small book that just contains the story. Does yours contain historical explanations in more details? Because you have a thick book! $\endgroup$
    – lucas
    Jul 8, 2016 at 3:23

Get a microscope slide or equivalent thin piece of clear glass. Paint the central portion with india ink or spray with black paint. The coating should be thick enough to prevent the passage of light. Let dry.

Using a razor blade and a thin straightedge, scratch a thin line through the paint across the slide (perpendicular to the long direction). As close as you can, scratch another line parallel to the first. Aim for a spacing of about 0.1 mm or less without disturbing the paint in between the lines.

Tape a piece of smooth white paper, the screen, to a box to keep it vertical on a table top. Do the same with an edge of your slide such that the slits are vertical with the long edge of the slide resting on the table. Start with the slide about 10 cm from the screen. Darken the room and shine your laser through the slits at the screen. Once you see some light on the screen, move the screen farther away to visually resolve the interference pattern. The darker the room the better.

This may take a few trials of scratching parallel lines to get a good pattern. Also, there are lenses you can put in front of your laser to make this easier.

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    $\begingroup$ A good way to get the two slits parallel and very close is to sellotape two razorblades together . The space between the cutting edges is then about the thickness of the blade. Then you cut with both blades simultaneously and should achieve two close, parallel cuts in one movement. $\endgroup$ Jul 8, 2016 at 8:50

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