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
What you need is:
- a small sheet of paper
- sharp scissors
- a black marker