A cheap and Easy Way to do the Double Slit Experiment
By Scott Little
Introduction
I am always looking for an easier and hopefully less expensive way to do things. Some of
my favorite things to experiment with are the properties of light. I was Googling around
and found a web site URL(1) that explained a very simple way to do the famous Young
Double Slit Experiment. It was first performed by Thomas Young in 1803 when he
presented his findings to the Royal Society of London. By asserting that light has both
the properties of a wave as well as a particle, he helped lay the foundation for what is
known today as Quantum Mechanics. This experiment is one of the most duplicated in
all of Physics, and it is considered by many to be the most elegant.
In its’ most basic form, the Double Slit Experiment consists of directing a beam of light
through a pin hole, then through two slits in a sheet of opaque paper, then finally
projecting the beam onto a screen of some sort. During the time of Young, there were no
forms of non-natural light, so what he did was direct a beam of sunlight through a hole
in a window shutter, then through a plate of “looking glass”, and finally split it in two by
a horizontally held card. The card was around 1/30 of an inch, a good approximation of a
modern-day 3”x5” index card. What formed on the screen were a series of interference
patterns resembling a wave hitting an obstruction. When dealing with the properties of
light, we say that it has been interfered constructively, which means that one crest of one
wave meets a crest from another wave (2). An image of such an interference is shown
below.
(1) Fig. 1. image courtesy of Scott Little
Mathematically speaking, we can demonstrate the interference with the following
equation:
λη/d=x/L
where
λ=the wave length of light,
d= the separation of the slits,
η=the order of maxima observed, central=0
x= distance between with bands of light and the central maximum-fringe distance
L= the distance from the slits to the screen
Even though this equation is an approximation, it still can be used to calculate some of
the quantities important to the Double Slit Experiment. For example, if you do not know
the wave length of the light, but are able to measure the separation of the slits, the fringe
distance from the center and the edge of the visible light (which will also give you the
maxima) and the distance form the slits to the screen, you can determine the wavelength
by λ= dx/Lη.
My Version
I decided to build my own apparatus. The materials I used were the cheapest and most
simple I could possibly find. Most of these items, with the exception of the wood
mounting, can be found at the average stationary store and cost less than $20. I used a
standard laser diode pointer at 630-680 nanometers, which is the wave length of red light.
A word of safety: please never look directly into any laser beam, even one as weak as
this.
For the card I used a standard 3”x5” stationary card, and for the pin hole a piece of
cardboard with an approximate 1/8” hole in the center. I have the actual bill of material
including dimensions at the end of article. As with most all of my projects, I always try to
include my kids and have an element of teaching involved.
We set up the experiment as follows:
Distance from the experimenter end of the board to the pin hole card: 6 3/4”.
Distance from the laser position to the pin hole card: 3”.
Distance from the far edge of the 3”x5” card to the screen: 7”
We position the screen close enough to be able to see the light projected, but far enough
away to create some form of interference pattern. Here is a diagram of the apparatus
showing all the measurements set up for the experiment.
Fig.3 courtesy of Scott Little
I secured the 3”x5” card using a standard paper clamp, and wrapped rubber bands around
it to hold it in place. For the pin hole card I used black electrical tape on both sides to
hold it upright. There are also screws protruding form the bottom of the board at the
mounting points of the pin hole and card that I used for a base.
The most difficult part of the experiment was to keep the card in the correct position with
the pin hole, which required constant adjusting. Another difficulty was the fact that we
held the laser in our hands and did not have a mounting for it. This was necessary to hold
activation button down.
Knowing the wave length of light, measuring the distance from the slits to the screen, and
approximating the fringe distance and the maxima, I used our equation to determine the
separation of the slits. With a little Algebra I got:
d= ληL/ x
λ=the wave length of light= 630 NM.
η=the order of maxima observed=1 approx.
x= fringe distance= 6” approx.
L= the distance from the slits (pin hole card) to the screen=7”
Now all the units need to be uniform, so I converted everything to meters:
1 inch.=0.0254M
1M=3.28 ft.
d= ληL/ x= (630x10-9)(1)(7x.0254)/(6x.0254)=7.35x10-7M=
2.9x10-5in.
This seems like a small number, but the screen
was close and the order of maxima was 1.
Other derivations can be performed with this experiment, and there are literally hundreds
of different versions of the Double Slit Experiment existing today. Many utilize
expensive equipment and measure the path of one photon going through the screen at a
time. In fact, in 2002 a more modern version of performed by Claus Jönsson was voted
the “most beautiful experiment” by the readers of Physics World (2). These versions may
be out of reach to average Citizen Scientists, but we can still duplicate the original in its’
elegance and simplicity.
Bill of Materials
(1) Plywood board: 15”Lx 2-1/2”Wx1/2”Thk. Nails placed at mounting points 6-3/4” for pin hole, 1/2” for 3”x5” card from same end.
(1) Laser diode pointer: 630-680 Nm wavelength Class 2, <1mW.
(1) Stationary note card: 3”Wx5”L x1/30”Thk.
(1) Cardboard pin hole: 4-1/2”Wx3-1/2”Tall; 1/8’ hole at 2”x2” center.
(1) Screen: 20”Wx30”Lx3/16”Thk white foam board. Purchase at art supplies store.
References
(1)http://www.cavendishscience.org/phys/tyoung/tyoung.htm
(2)1http://en.wikipedia.org/wiki/double-slit_experiment.