How I made my own apparatus for the double-slit experiment.

Be aware that if you make one of these devices for yourself you must follow the manufacturer's rules for laser use (unless you use the pre-laser method involving a pinhole admitting sunlight into a darkened room, etc.)  because you could burn a portion of your retina by looking into the beam. You are entirely on your own. I only explain how I made something for my own use.  

1. I got a plastic railway track section from a toy store. I could have made a similar frame from chopsticks with a square cross-section.
Picture of a toy plastic railroad track unit

I found these little track segments in a "dollar store" toy section. I got a package of several of these plastic units (shown here at about life size) for one dollar. I was a little surprised to discover how sturdy they are.



2. I cut the plastic track down to a suitable length.
Picture of a short segment cut from the full track unit

Only a couple inches of track are needed to make a support for the double slits, so I cut off a short piece using a fine-toothed hack saw blade (white painted metal strip shown near the top).


3. I made a rough opening in which to place the bars that will form the two slits.
Use chisel to make rough opening for the slit apparatus.

I used a steady heavy pressure on the handle of a wood chisel of appropriate width, which allowed me to cut out a couple of the "ties," but the cuts were not flush with the rails.



4. I smoothed the edges of the opening.
Use hacksaw blade to cut off pieces projecting into opening.

It is difficult to find tools small enough to do a perfect job. I used the same hacksaw blade mentioned above to saw out the little left-over parts of the "ties" that I cut out with the chisel. A small file would have done a neater job.



5. i arranged three pieces of automatic pencil lead, and then taped one end of the three pieces and fastened them with JB WeldⓇ. In the past I have used faster-drying adhesives with success.
Position leads in flea comb and tape opposite ends

Getting three automatic pencil leads lined up properly is not easy. All three must be parallel and they need to have equal spaces between them. I tried various ways and ended up pushing the ends of the three leads between adjacent teeth of a comb designed to remove fleas from the coats of cats. I had to clamp the blue plastic handle of that comb in a vice to make it hold still for me. Then I held the other ends together and stuck them to some transparent packing tape. (I had to roll the tape into a cylinder and put one finger inside in order to be able to control the pencil leads and the tape with two hands.) Once I had them in approximately the correct position, I mixed a little  JB Weld adhesive and dabbed some onto the ends that were stuck to the tape. JB Weld takes about four hours to dry, but it is extremely strong and I could still move things around while it was drying. Other products dry faster and most should be adequately strong, but JB Weld was what I had at hand



6.  I put a dab of glue on the taped end.
Dan glue on the taped end.

It's difficult to see the grey glue on the ends of the leads where they are held by the tape, but it's there.  I made sure to let it dry thoroughly before going on to the next step.



7. I taped the other end of the pencil leads and then put a dab of glue on them.
Tape and glue the other ends of the leads.

I next held the leads near the flea comb end and stuck them down to another cylinder made of transparent packing tape. Following that, I put glue on that end. At this time I made a few adjustments necessary to get the spacing between the pieces of pencil lead right. I could have used slips of paper to force them farther apart if it had been necessary, but this time I got lucky. (The slits in the device I made are approximately 0.01 inch apart.) Then to both ends I applied a fairly large drop of adhesive to stabilize the pieces of lead in the correct relationship to each other. Because JB Weld dries relatively slowly I took a look at my project from time to time just to make sure that nothing had slipped out of place. I wanted to have a very stable bond once everything was lined up properly, a bond that would not pop loose somehow and spoil all my efforts.



8. I positioned the leads on the track track and and then put a dab glue on one end.
Place leads on track and put a dab of glue to join them.

I found the part of the three-lead unit that had the best spacing characteristics, and fixed the unit to one of the horizontal members with a dab of adhesive so that the best part would be within the tracks.



9. I straightened the three-lead units and dabbed glue on second end.
Make leads perpendicular to "tracks" and dab with glue.

I made my best attempt to line up the three-lead component so that it was perpendicular to the plastic "tracks." Only then did I put a dab of adhesive down to join the leads to the second "track."



10. I glued both juncture points securely to the track once the position was idealized.
Glue leads securely to "tracks"

I appplied plenty of adhesive once the components were positioned correctly because I was getting ready to clip the excess lengths of pencil lead, and I did not want to break anything except the excess lengths. When the bonds were solid I grasped the pencil leads with a forceps just inside inside the part that I wanted to remove. I held the forceps tightly and snapped off the unwanted end. then I repeated this operation on the other side.



11. I positioned small nails to act as vertical mounting supports, and I weighed them down with larger nails, and dabbed the small nails with adhesive at their junctures with the tracks.
Position and glue vertical supports

I first turned the plastic "railway track" over. I glued two small nails (about twice as long as the track is wide) to the edges of the "window" in which the pencil leads had been glued. They looked like they might slip around so I weighed them them down with much heavier nails to keep them in the proper position while the adhesive was hardening. I tried to make very sure that the vertical supports were mounted at 90° to the track before the glue set. Once I had everything positioned correctly I added enough adhesive to create a very strong bond.



12.  I turned the apparatus over again and taped up the open spaces in the "window."
Make shutter tapes on both sides.

It is important to cover the outside pencil leads to their middle points, but not any further. So I slowly slid the strips of tape from the outside toward the inside, and then let them adhere to the tracks when they had just closed the gaps between the outside edges of the leads and the large empty spaces in the frame.



13. I turned the whole thing over again and glued this new tape to the back side of the tracks, just to be sure it would not come off.
Glue tape to track

Electrician's tape has good adhesive qualities, providing that the adhesive surface is not contaminated with powder, oil, etc. However, having gone to a fair bit of trouble to line eveything up I did not want the tapes to fall off, so I put some JB Weld on the back side to try to ensure that  the tape will stay permanently joined to the rest of this apparatus.  It was difficult to get the adhesive to go on smoothly, but I was much more concerned to bridge the gaps between the outside nubs of the "ties" so that the tape would be attached in an unbroken line all along the parts outside rails.



14. I trimmed off the excess tape and adhesive.
Trim off excess adhesive and tape

I used ordinary scissors to cut through the tape and the thick layer of adhesive, leaving the edges relatively even.



15. I made my own arrangements to secure a laser and learn how to use it safely. All laser devices carry warnings not to look into the beam. The reason is that the laser light can burn a hole in your eye, sort of the way that a magnifying glass can direct the light of the sun onto something and burn it. I believe in being cautious, so I sometimes use a pair of special safety glasses designed for the specific frequency of the laser light. Because the double-slit part of the apparatus I made is all black I felt it was relatively safe compared to an earlier version that used shiny brads. Because they were reflective, a beam from the laser might get bounced directly into my eyes. I did not want to take chances. I knew that I should not view any beams coming from the laser itself -- even through the double-slit apparatus.



16.  Once I had turned on and aimed the laser, I saw the interference (shown in the photo below) projected on a white screen a few feet away.

Interference pattern produced by this device

I viewed this interference pattern on a white matte surface to avoid the possibility of direct reflections from the laser. (See Laser Safety on Wikipedia.) Lasers come in different strengths. Anybody who uses a laser should follow the warnings on the laser. On my laser it says: "Laser light is harmful to eyes! Do not look directly into beam!"


Anyone who wants to make experiments using lasers is entirely responsibility for their own safety. The author of this page takes no responsibility for the acts of others, and offers no claim of safety. To the contrary,

Always take all due precautions when using lasers. Laser light is harmful to eyes! Do not look directly into a laser beam!

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