Wave-Particle Duality

1
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.
Results
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.
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,
?, of the light(2) dsin? m? Youngs 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. 8
Eq. 1
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. Einsteins 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.
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
• http//www.assignmenthelp.net/assignment_help/youn
  g-double-slit-experiment.phpz
• Hecht, Physics Algebra/Trig, Brooks/Cole (1998)
• Anderson, Introduction to Modern Physics,
  Saunders College Publishing (1982)
• Feynman, Leighton, and Sands, The Feynman
  Lectures, Vol. III, Addison Wesley (1965)