This 'ible will show two simple methods to make a double-slit, that is small enough to create an interference pattern.

Interference is an effect waves show, when they interact with each other. If two (or more) waves have the same wavelength and a fixed phase relationship they are called "coherent". And if such coherent waves hit the same spot, they can support each other, annihilate each other (yes, you did read right: light + light = more light or no light) or something in between depending on their phase difference. The special thing about laser light is its huge coherence length meaning the light emitted by a laser is practically always coherent. A double-slit (or better a lattice) splits up the laser beam into two (or many) "beams". Beyond the double-slit or lattice each new "beam" spreads in every direction interfering with the other one(s). This creates a characteristical pattern of maxima and minima: the interference pattern. For further information have a look at the wikipedia articles about the double-slit experiment and interference.

Disclaimer: This instructable presupposes responsible handling of laser sources. DO NOT DIRECTLY LOOK INTO ANY LASER BEAM. Always handle laser beams with caution. I'm not responsible for any injuries (e.g. damaged or blinded eyes) due to wrong handling of your laser source.

Step 1: Double-slit Out of Paper: Materials

We'll start with a fast, but pretty low quality double-slit out of paper.

What you need is:

a small sheet of paper
sharp scissors
a black marker

Step 2: Double-slit Out of Paper: Making

Take the marker and paint a small area black in the middle of the upper half. This is where we'll cut two slits into the paper. The black paint will prevent the laser from shining through the white paper.
Fold the paper horizontal at the half of the black area.
Now cut from the edge a perpendicular slit into the black area. Then try to cut as close as you can parallel to the first cut and rip out the piece of paper in between to create a narrow slit. Repeat the same about half a millimeter next to the first slit.
Unfold the paper and have a look at your double-slit. If it doesn't have a nice straight shape, start again with 1.

Step 3: Double-slit Out of Aluminum Foil: Materials

Aluminum foil allows us to cut a lot more precise and closer slits. This will create a cleaner and wider interference pattern, because the distance from the middle ( called "0. maximum") to the outer maxima ( 1. / 2. / 3. / ... / n. maximum ) increases the smaller the distance between the slits is.

For the double-slit out of aluminum foil you need:

a small sheet of paper
a piece of aluminum foil
an x-acto knife
sticky tape
a magnifying glass

Step 4: Sticking the Foil to the Paper

Roughly cut out a small rectangle from the sheet of paper.
Lay it down onto the table and place the foil onto the rectangle. Now flatten the foil with your finger, so the edges of the rectangle leave marks in the foil.
Cut off all the excess foil, but leave a bit of a frame.
Tape the foil to the paper under a bit of tension. The less reflective side should be visible from the other side trough the "window" in the paper.

Step 5: Cutting Slits Into the Aluminum

Use the x-acto knife to cut two straight slits as close together as possible without damaging the tiny strip of foil between them. A magnifying glass is very helpful to cut more precisely.

Step 6: Using the Double-slit

Tape your double-slit to the edge of a table and aim with a laser beam at the double-slit. I advise you to aim with low intensity (if you can adjust it), since you don't want to look into the bright reflection of the full-power beam too long. Then darken the room and use a sheet of paper, cardboard or a white wall as your screen. You should see the characteristical interference pattern of maxima and minima now.

Thanks for reading and I would appreciate some pictures of your interference patterns!