double slit experiments is light a wave or particle? is an electron a wave or a particle?
TRANSCRIPT
Double Slit Experiments
Is light a wave or particle? Is an electron a wave or a particle?
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GUN
If you had a gun and fired a bullet perfectly straight at the spot denoted by X you would expect the bullet to hit that very spot for short distances.
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If there was a slit in the object you would expect the bullet to fly straight through it and strike the wall directly behind it where the X is located.
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If there was a slit in the object you would expect the bullet to fly straight through it and strike the wall directly behind it where the X is located.
X
Instead of using a gun and bullets, what happens if we shoot electrons or light through the slit?.
GUN
If there was a slit in the object you would expect the bullet to fly straight through it and strike the wall directly behind it where the X is located.
X
Instead of using a gun and bullets, what happens if we shoot electrons or light through the slit?.
We would expect the same thing to happen if an electron is a particle. It should strike the X.
X
ElectronGUN
Now lets say we shoot a bunch of electrons one at a time through one slit at target X behind it which also has two slits in it.
X
ElectronGUN
Now lets say we shoot a bunch of electrons one at a time through one slit at target X behind it which also has two slits in it.
If the “wall” is a screen that can detect electrons then something very curious happens.
Detector Screen
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ElectronGUN
The detector screen behind the second slit
detects electrons which are denoted by dots
above.
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ElectronGUN
The detector screen behind the second slit
detects electrons which are denoted by dots
above.
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Behind the second slit on the right
electrons are also detected by the
screen.
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Not only do we detect electrons behind the slits,
but there is an interference patter as well which we will
see in a bit.
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This does NOT make sense if an electron is a particle. If an electron is a wave then
this does make sense!
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ElectronGUN
Single Slit
Double Slit
Detector Screen
If Light or Electrons behave as waves when we pass them through a slit they will “reemit” at the source”.
ElectronGUN
Single Slit
Double Slit
Detector Screen
Notice how the waves start overlapping after passing through the second slit (not drawn to scale)!
Anatomy of a Wave
Typical Sine Wave
Crest
Trough
Wavelength (λ): distance between the waves (from crest to crest or trough to trough).
AmplitudeWave Height
Wave InterferenceConstructive Deconstructive
+
=
When two waves come together and the crests line up, we end up with a bigger amplified wave.
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=When two waves come together and a trough and crest line up, the waves cancel one another.
ElectronGUN
Single Slit
Double Slit
Detector Screen
The waves may constructively or deconstructively overlap before reaching the detector screen and this leads to a pattern of interference.
ElectronGUN
Single Slit
Double Slit
Detector Screen
In this pattern the white areas are where electrons are detected (the waves are amplified) and the dark areas represent regions where
electrons are not found (waves that cancelled out).
Young’s Double Slit Experiment
Where the waves are amplifying with one
another.
Where the waves are cancelling with one
another.
Light/Matter as waves
• The wave nature of light (and electrons) has been demonstrated by slit experiments and interferometers.
• Thomas Young (1801) is credited with the first slit experiment that led to the acceptance of the wave nature of light. The wave nature of electrons was discovered in the early 1900s.
IMG SRC: Wiki Commons
Two Videos to Watch
Dr. Quantum Double Slit Experiment
Greene : What's Beyond The Double Slit Experiment ?
ElectronGUN
Detector Source used to observe which slit the electron passes through
The act of observing the electrons collapses the wave function!!!!
ElectronGUN
Detector Source used to observe which slit the electron passes through
When you observe the electrons at the interface
of the slit, they end up behaving like particles
The act of observing the electrons collapses the wave function!!!!
ElectronGUN
Detector Source used to observe which slit the electron passes through
When you observe the electrons at the interface
of the slit, they end up behaving like particles
The act of observing the electrons collapses the wave function!!!!
Quantum mechanics tells us that the act of observing
reality changes it!
The Observer Effect
Photon
• If you wanted to know where an electron was located or its momentum you would have to observe it. The act of observing an electron requires a photon to bounce off of it which changes its location/momentum.
• A macroscopic example is the difficulty in determining tire pressure without letting any of the air out.
Electron
The Entanglement of Science and Consciousness
Young’s Equation λ = y • d / (m • L)
• λ = wavelength • y = distance between the bands being measured. • d = slit separation distance. • m = order magnitude or number of bands. • L = distance from slits to detector screen.
Lightd
L
-3
-2
-1
0
1
2
3
m
y
A 0.25mm slit (d) is separated from a screen by 9.7m (L) The distance between the central bright band and the third bright band is 7.5cm (y). What is the wavelength of light?
Young’s Equation λ = y • d / (m • L)
• A 0.25mm slit (d) is separated from a screen by 9.7m (L) The distance between the central bright band and the third bright band is 7.5cm (y). What is the wavelength of light?
WHAT DO I KNOW?• d = 0.00025m• L = 9.7m• y = 0.075 m• m = 3
Young’s Equation λ = y • d / (m • L)
• A 0.25mm slit (d) is separated from a screen by 9.7m (L) The distance between the central bright band and the third bright band is 7.5cm (y). What is the wavelength of light?
WHAT DO I KNOW?• d = 0.00025m• L = 9.7m• y = 0.075 m• m = 3
λ = y • d / (m • L)λ = 0.075m • 0.00025m / (3 • 9.7)
λ = 6.44x10-7m = 644nm (visible)
Problems λ = y • d / (m • L)
1. A student uses a laser and a double-slit apparatus to project a two-point source light interference pattern onto a whiteboard that is located 5.8 meters away. The distance measured between the central bright band and the fourth bright band is 8.2 cm. The slits are separated by a distance of 0.15 mm. What would be the measured wavelength of light?
2. Two slits separated by 0.250 mm produces an interference pattern in which the fourth bright band is located 10.8 cm from the central bright band when the screen is placed a distance of 8.2 meters away.
3. An interference pattern is produced when light is incident upon two slits that are 50.0 micrometers apart. The perpendicular distance from the midpoint between the slits to the screen is 7.65m. The distance between the two third-order bright bands of the pattern is 32.9 cm.
4. Is a baseball being thrown down a hallway a wave or a particle? Your answer should include a mention of a famous French physicist named Louis de Broglie.