Two Slit Diffraction & Interference Demonstration
A photo essay.

Updated: 060110 - Views:
All content copyright © 2003-2009, B Vanderkolk

In this section I will show you how you can perform your own demonstration of the interference patterns from diffraction of two closely spaced slits using easily available materials. This demonstration can be easily done in an hour.

In this first picture is everything needed to perform the two slit demonstration:
A piece of window glass

  • Two brand new razor blades
  • Clear plastic tape
  • Some water based acrylic paint. Choose a color that absorbs the color of your laser. In my case I chose some dark forest green since I am using a red HeNe.
  • Of course, a laser.
The first step is to apply two parallel strips of tape to the clean glass, leaving a band of glass uncovered. Rub your fingernail along the tape to make sure there are no bubbles along the edges. The gap should not be wider than your razor blades.
Apply a good sized drop of paint towards one end of the bare strip.
Take a razor blade and spread the paint along the strip. The razor blade should rest across the tape. This keeps the blade above the glass the same thickness as the tape allowing you to spread a constant thickness of paint along the strip. You may have to experiment with the amount of paint and the amount of pressure so you get a nice smooth thin layer of paint. You want the paint layer thin enough to score with the blades yet thick enough to absorb the laser light.
Carefully remove the tape and allow the paint to dry. The water based acrylic craft paint I used dries in an hour or less.
While the paint is drying, stack the two blades together. Be sure to use clean blades.
Use some folded paper towards the back side of the blades to seperate them. This forces the shapr edges of the blades closer together. To make sure the blades are even place them against a hard surface to align them. You may have to experiment with the amount of spacer material in order to adjust the blade seperation to get the blades close enough together.
Tape the blades together. Leave at least one corner uncovered as this is the part that you use in the next step. You want the blades to be as close together as possible but if you get them too close they wont score two seperate lines in the paint but instead make one fatter line.
Use the two parallel blades to score a pair of lines across the paint. Once again you may have to experiment with how much pressure to use. You should score with a brisk motion. You want to score two parallel lines leaving a thin line of paint to seperate them.
You can see the scored lines in this picture but the resolution is too low to see the seperate lines clearly.
Set up the paint strip in front of your laser. Adjust the postion of the slide so you get the maximum amount of light through the slits.
On a far wall you should get an interference pattern. This image was taken twelve feet from the slits with a one foot ruler for comparison. The number and spacing of spots will vary depending on the quality and spacing of the slits.