This experiment will show how physicists in the 20th century argued that light behaves as a wave. We will reproduce this experiment not only in a very easy-to-do and affordable manner, but in a fairly original way. So by following the simple instructions, not only will you be wowing any spectators with the magic of physics, but you will be re-creating one of the most important experiments in the history of physics.
Step 1: Gather Materials
You will need:
A relatively strong laser pointer
A piece of three holed paper
Batteries for the laser pointer
A bare wall
A flat surface
A long piece of hair
Step 2: Prepare Double-Slit
Tape the piece of hair on the paper, as shown in the image, over a hole in the paper. Be sure to tape the hair down so that the strands are very tight and close together, or the experiment will now work too well.
Step 3: Prepare Laser
Tape the button on the laser pointer down so that the light never goes out. Now find a flat surface that has a clear view of the wall. Now tape the laser down to the flat surface so that the laser is hitting the wall. Since the laser may be on for a while while you tinker and prepare, you may need to periodically change the batteries in the laser.
Caution: Laser pointers can cause serious damage to eyes or cameras, so be careful and responsible.
Step 4: Create Diffraction Pattern
Hold the paper that you made in step 2 in about a foot in front of the laser pointer. Hold the paper so that the beam of light is hitting both pieces of hair. Bend the paper to tighten the hair if you need to. This should, if you are performing this step properly, create a diffraction pattern of the wall the the laser is pointed at.
Step 5: Explain
This is a cool experiment, but why does this happen? This experiment was used in the early 20th century to prove that light behaved as a wave. We know now that light behaves in a way that we call a "wave-particle duality". This basically means that light behaves as both a wave and as a particle, the particles being photons.
What happens here is the hair causes the waves of light to separate as the waves go through. Once the waves pass the hairs, they begin to bounce off of each other, just like water does. The bouncing of the waves is called "diffraction" and causes a "diffraction pattern" on the wall.
Step 6: Cost
This experiment was extremely cheap. The laser pointer was borrowed, but can be bought on amazon for around $20. The tape I used was bought for $1. The paper was in a $1 notebook. And the hair was from my little sister. So the total cost of this experiment is about $22.
Step 7: Looking Back...
If I could re perform this experiment, better materials would have helped. I spent some time searching for something to use other than hair, but now luck was there. The physics teacher at my school had recommended using carbon powder on a glass slide to perform the experiment. This would have worked phenomenally, but carbon-powder was not the easiest, nor cheapest thing I could find. Overall, not much would have made the experiment better. It is cheap and easy to do, using different materials, like carbon powder, would have caused much searching and more money than what needed to be spent.