Double Slit Experiments

Is light a wave or particle? Is an electron a wave or a particle?

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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.

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Instead of using a gun and bullets, what happens if we shoot electrons or light through the slit?.

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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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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.

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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.

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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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The detector screen behind the second slit

detects electrons which are denoted by dots

above.

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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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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”.

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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

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=

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.

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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.

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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 ?

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Detector Source used to observe which slit the electron passes through

The act of observing the electrons collapses the wave function!!!!

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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

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0

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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.