Physics Group: Alex Pearson, Travis Christolear, Mei Mei Chan, Colin Smith
Faculty Advisors: Vladimir Gasparyan, Thomas Meyer
Objective: The properties of light have been variously explained by viewing light as a particle (Newton, Einstein, Compton) or as a wave (Huygens, Fresnel, Young, Maxwell). The
main goal of this experiment was to study the quantum mechanical resolution of these two different viewpoints by demonstrating the wave-particle duality of light.
Wave Nature of Light
Experimental Apparatus
In 1801, Thomas Young projected light through two slits onto a screen. His
experiments showed that light behaves as a wave, putting into question preconceived
Newtonian notions that light was a particle. In this experiment, each slit acts as a
source of light, and the resulting pattern was explained using the constructive and
destructive interference of waves.
Results
Fig. 3 The Apparatus
Fig. 7
Fig. 1 Double Slit
Interference(1)
The interference pattern created by this experiment produces bright maxima (Fig. 6)
where the path length difference between the two slits is equal to an integer number,
m, of wavelengths, λ, of the light(2)
Eq. 1
dsin = m
Young’s experiment was reproduced using a diode laser (=670nm) as a source of
coherent light and a photodiode that measures the intensity of the light across the
interference pattern. Our results confirm the wave behavior and give the distance, d,
between the two slits of 0.419mm.
Fig. 4 The Photon Counting Assembly
Fig. 8
Fig. 6 Single and Double Patterns
The Oscilloscope
Fig. 5 Double Slit
The
Detector
The
Digital
Counter
Fig. 2
Wave-Particle Duality

According to Richard Feynman, wave-particle duality "contains the only mystery
of Quantum Mechanics"(4). In this experiment we demonstrate the wave-particle
duality for photons. When light passes through a double-slit the characteristic
interference pattern is observed, which can be explained by classical wave theory.
When the intensity of light is reduced to such a low level that only one photon
arrives at the two slits classical physics predicts that the photon must choose to
pass through one or the other slit.
Particle Nature of Light
By the end of the nineteenth century, there were results that could not be explained
using the wave theory of light. Einstein’s Nobel Prize winning explanation of the
photoelectric effect(3) led to the reemergence of the particle theory of light. Our
experimental apparatus (Fig. 3) allows us to perform measurements using single
photons. The source is a variable light bulb with a selective filter that allows only
green light and can have its intensity lowered to the point that only a small number
of photons are passing through the apparatus at a time. The detector is a highly
sensitive Photo-Multiplier Tube (PMT) that amplifies the signal from a single photon
to a voltage that can be measured with an oscilloscope. Each signal is also output as
a single TTL pulse which is electronically counted (Fig. 4). The speed of light
(3x108m/s) is great enough that if photons were sent one at a time through the
apparatus (approximately 1m in length), more than 300 million counts could be
registered per second. When the experiment was performed, we collected an average
of 400,000 counts per second. The PMT is only ~ 2-3% efficient, so this gives an
actual rate of approximately 1 million counts per second, or 1/300th of that minimum
rate. This gives confidence to the claim of the ability to perform the experiment with
one photon at a time.
Troubleshooting
While conducting various trials and experiments, an asymmetry was observed in
the graph for the double slit interference pattern (Fig. 7) using the light bulb and
the PMT detector, but not seen with the laser and photodiode detector. To study
this asymmetry the double slit was reversed, inverted, and displaced. The results
remained unchanged. After a series of other changes and additional trials, it was
concluded that the PMT efficiency was not uniform across its surface.
Single Photon Experiment
The results of the low intensity light bulb (λ=546nm) were plotted as a function
of the detector slit’s positioning along the photomultiplier tube. The PMT output
was measured using an electronic counter and then averaged over 30 seconds for
each detector position measured in 0.05mm increments. The experiment was then
repeated with each of the two individual slits covered to show that the light from
the double slit was not just the sum of the light from the two individual sources
(red and green curves in Fig. 7). To emphasize this result we plot in Fig. 8 the
counting rate at the location of the central maximum and the adjacent minima.
Classically, we would expect the counting rate at the center to be the sum of the
counting rates for the individual slits, or about twice that at the adjacent minima.
However, we observe a counting rate which is about three times larger.
Using the equation from Young’s experiment (Eq. 1) the separation between the
two slits was calculated to be 0.42mm.
The main difficulties in this experiment arose when trying to align the light
source, slits and detector, especially using the low intensity light. The random
nature of when and where a photon would land on the detector gave a wide range
of counting rates that had to be averaged over time.
Conclusion
The two experiments, using a high intensity laser beam and single photons from
a low intensity light bulb passing through a double slit set-up, produce nearly
identical results: a double slit interference pattern measuring the distance
between slits of 0.420mm and 0.419mm, respectively. In the case of single
photons, this leads us to conclude that a single light particle, or photon, is also a
wave, which passes through both slits at the same time.
References:
1) http://www.assignmenthelp.net/assignment_help/young-double-slitexperiment.phpz
2) Hecht, Physics: Algebra/Trig, Brooks/Cole (1998)
3) Anderson, Introduction to Modern Physics, Saunders College Publishing
(1982)
4) Feynman, Leighton, and Sands, The Feynman Lectures, Vol. III, Addison
Wesley (1965)