DIY Interferometer: Prove Einstein Wrong in 20 Minutes!
by DrewPaulDesigns

What is light? Is it a particle or is it a wave? What an imprudent question! As we know, these are not mutually exclusive characteristics. “Particle” describes the composition of a substance and “wave” describes the motion of it. Right?
But…, wait…, so…, that still doesn’t answer the question… Is light a particle or a wave? Let’s find out!
Today, we're going to add a new piece of equipment to our laboratories and perform a mind boggling experiment with astronomical implications!
First, we will be constructing an Interferometer, a useful piece of experimental optical equipment capable of performing a variety of tasks in a laboratory setting including the Michelson-Morley Experiment, the Sagnac Experiment and others. Specifically though, we will be building a common path Interferometer with an interesting variation. Our version will be taking the approach of physicist Robert Young's 1801 Double-Slit Experiment so our Interferometer will split one single beam of light into two by passing it through two small openings.
Next, we will examine characteristics of moving particles and those of waves moving through a substance. We will then use our new Interferometer to perform the Double-Slit Experiment ourselves. Next, we will observe the result and compare it to what we learned in our previous experiments. You’ll be astounded at what you find!
Let’s begin!
Step 1: What you'll need
Before we get started on this project, there's a few things we'll need. Unlike the experiments of the 19th and 20th centuries, our project can be made from now easy to find materials and built in a way where the experiment can be accurately replicated anywhere and performed a multitude of times with ease.
Robert Young found that the sun would be the optimal light source for his experiment. And it was, ...at the time. See, for the purposes of what we're going to be doing today, just as in his Double-Slit Experiment, we need parallel bands of light for comparison purposes. The problem is that most light sources emit their light radially, out in all directions, which criss-cross. The sun does too, but since it is a great distance away, the sun's rays are nearly parallel. (Think: a radius of 1cm makes a sharp curve but a radius of 1km looks almost like a straight line.) So, the sun is a fairly good light source, but a laser is better! A laser beam is a highly parallel light source which is what allows it to travel a large distance and still appear as a small dot.
PARTS:
(1) Laser- 630-650nm, <5mw, 4.5v
(1) Project Box
(2) Insulated Wire
(1) 3AA Battery Holder with Switch
(3) AA Batteries
(1) 32AWG Copper Wire
(1-2) Wire Connectors
(1) Pvc Pipe with Cap
(1) Plastic Ramekin or similar item
Shrink Wrap Tubing
TOOLS:
Drill
Large Drill Bit
Small Drill Bit
Sanding Bit
Hole Punch Bit
Soldering Iron & Solder
Scissors
Glue
Various Tape, Masking and Aluminum Tape or Electrical Tape

Make due with what you have on hand if need be. I painted a metal cookie box black for the enclosure, a small plastic dish or bottle cap works in place of a ramekin, inexpensive lasers are typically more than sufficient. Feel free to get creative with the rest of the parts as well.
*Read warnings and take cautions when working with lasers.

Step 2: Prepare your battery pack
Heat your soldering iron and strip the ends of the Battery Holder leads and Insulated Wire. Slip on some shrink wrap and extend the leads by soldering on a length of wire to each. Before you're finished, insert your batteries and lightly tin the ends of the wires to prepare for the next connection.
If you don't have any AA Batteries, MAKE YOUR OWN!
See how by checking out my last article: DIY AA Batteries!

Step 3: Wire your laser
To wire your laser we will need to make a quick modification. In order to take data while experimenting, we'll need to leave the laser on for extended periods of time. This means we'll need to replace the button with a switch and replace the batteries with some of a higher capacity. The Battery Holder we just prepared will serve both these purposes. We'll just need to connect it.
To access the leads we'll first remove the Laser's battery cap and remove the batteries. Next, grind down one side of the Laser's case, as shown, or cut it away however you see fit.
According to the original battery orientation and the diagram, the spring is the negative ground and the case will serve as the positive lead for the laser. With your soldering iron attach one wire to the spring and use a piece of shrink wrap to secure and cover the spring to avoid shorts. Then, drill a small hole in the case, wrap your positive wire around securely, solder in place and shrink wrap that connection as well. Connect the ends of each wire to your Wire Connector(s).
Your laser module is now complete and ready to be temporarily wired up to test your connections. Onc