How to demonstrate interference or diffraction in class? High School physics teacher here. I was wondering about a simple experiment that I can perform in class to demonstrate interference or diffraction. Can you suggest anything?
I am not looking to actually make predictions, do measurement, verify result and perform rigorous experiment. Just a simple experiment through which I can say that - "THIS IS INTERFERENCE. Now we will study the details behind it."
 A: If you point a laser pointer at a sufficiently high resolution digital screen (anything made in the last decade should work), the reflected spot should be split into several dots; the spacing and arrangement of the dots carries information about the size and shape of the pixels on the screen. (This is a simplified variation of a technique researchers use to measure the molecular structure of crystals.) Lower-tech variations include:

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*Point a laser at the hologram on a credit card or on currency, or even just the shiny side of a CD. You should at least get the dots. You may also get a proper holographic image: some designs project readable letters or other 2D images via engineered diffraction when lit by a laser. (These images are not apparent when viewed normally; they only exist as diffraction patterns.) If you can find a good hologram, it can be quite striking: 

*Point a small flashlight at a high-resolution (newer the better) TV screen. You should get a pretty pattern of rainbow spikes. (These are colored and smeared versions of the dots you'd get from a monochromatic source.)

*Point a laser pointer at a plucked hair. Same general result (spaced fringes carrying information about the hair's size) as with using a screen, but the contrast between fringes is lower because the generating pattern is not repeated in space.

*Perhaps easiest of them all: point a flashlight at the shiny side of a CD or DVD. The wavelength dependence of diffraction from the patterns on the disc causes white light to split into its colors when reflected.

A: If you have access to an OLD SCHOOL OVERHEAD PROJECTOR, here's how to do it.
Set the projector up in front of a projection screen. Place a clear glass rectangular roasting pan (about the same dimensions as the illuminated top of the projector) on the projector and put about an inch of water in the pan. By dipping the tip of your finger or a blunt pencil in the center of the water you will project wave trains across the projector screen.
Using lego blocks, build a wall taller than the water level across the middle of the pan, with two gaps in the wall about 1" apart and about 1/2" wide each. make waves behind the wall; these will then escape through the two "slits" and produce a nice interference pattern on the other side of the wall.
You can also use this arrangement to demonstrate reflection off the wall and diffraction around a corner of a wall which extends only halfway across the pan!
