1. Double Slit Experiments
Is light a wave or particle? Is an electron a wave or a particle?

2. 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. X
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
GUN

3. 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
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
GUN

4. 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
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
Instead of using a gun and bullets, what happens if we shoot electrons or light through the slit?.
GUN

5. 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
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
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.
GUN

6. X
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
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.
Electron
GUN

7. X Electron GUN Detector Screen
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
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.
Electron
GUN

8. X .
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
The detector screen behind the second slit detects electrons which are denoted by dots above.
Electron
GUN

9. X . .
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
The detector screen behind the second slit detects electrons which are denoted by dots above.
Behind the second slit on the right electrons are also detected by the screen.
Electron
GUN

10. X . .
At longer distances, free fall due to gravity, Coriolis force, curvature of the earth, air currents, etc. must be looked at.
Not only do we detect electrons behind the slits, but there is an interference patter as well which we will see in a bit.
This does NOT make sense if an electron is a particle. If an electron is a wave then this does make sense!
Electron
GUN

11. If Light or Electrons behave as waves when we pass them through a slit they will “reemit” at the source”.
Electron
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Single Slit
Double Slit
Detector Screen

12. Notice how the waves start overlapping after passing through the second slit (not drawn to scale)!
Electron
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Single Slit
Double Slit
Detector Screen

13. Anatomy of a Wave
Crest
Amplitude
Wave Height
Typical Sine Wave
Trough
Wavelength (λ): distance between the waves (from crest to crest or trough to trough).

14. Wave Interference + + = = Constructive Deconstructive
When two waves come together and the crests line up, we end up with a bigger amplified wave. + + = =
When two waves come together and a trough and crest line up, the waves cancel one another.

15. The waves may constructively or deconstructively overlap before reaching the detector screen and this leads to a pattern of interference.
Electron
GUN
Single Slit
Double Slit
Detector Screen

16. 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).
Electron
GUN
Single Slit
Double Slit
Detector Screen

17. Young’s Double Slit Experiment
Where the waves are amplifying with one another.
Where the waves are cancelling with one another.

18. 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.
Light sources of the day (candles) were unreliable so Young used sunlight passing through a hole…
IMG SRC: Wiki Commons

19. Two Videos to Watch
Dr. Quantum Double Slit Experiment
Greene : What's Beyond The Double Slit Experiment ?‬

20. The act of observing the electrons collapses the wave function!!!!
GUN
Detector Source used to observe which slit the electron passes through

21. The act of observing the electrons collapses the wave function!!!!
When you observe the electrons at the interface of the slit, they end up behaving like particles
Electron
GUN
Detector Source used to observe which slit the electron passes through

22. The act of observing the electrons collapses the wave function!!!!
When you observe the electrons at the interface of the slit, they end up behaving like particles
Electron
GUN
Quantum mechanics tells us that the act of observing reality changes it!
Detector Source used to observe which slit the electron passes through

23. The Observer Effect Photon
The Entanglement of Science and Consciousness
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

24. 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. m -3 -2 -1 1 2 3 y d
Light
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? L

25. 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 = m
L = 9.7m
y = m
m = 3

26. 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 = m
L = 9.7m
y = m
m = 3
λ = y • d / (m • L)
λ = 0.075m • m / (3 • 9.7)
λ = 6.44x10-7m = 644nm (visible)

27. Problems λ = y • d / (m • L)
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?
Two slits separated by 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.
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.
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.