Young’s Double-Slit Experiment
• 1801, first to demonstrate the interference of
• Illustrates the wave nature of light
• See pattern of light and dark bands on screen
• Similar to water waves and sound waves
• Constructive interference à bright
• Destructive interference à dark
Pattern Spacing for Douple-Slit
• Rearranging, we get the pattern (fringe)
y = λLm/d
o y = distance from center of pattern to fringe of interest
o d = ______________________
o L = ______________________
o λ = ______________________
o m = _____________________
• If d increases, the pattern spacing (y) _______.
• If L increases, the pattern spacing (y) _______.
• If λ increases, the pattern spacing (y) _______.
distance from slits to screen
wavelength of light
0, ½, 1, 1.5, 2, 2.5, …
• Diffraction: waves spread out
• Light enters regions that would otherwise be
• Occurs when waves pass through small
openings, go around obstacles, or pass by
• Light going through a narrow slit similar to
• Each portion of the
slit acts like a
source of waves
• Light from one
portion of the slit
can interfere with
light from another
portion of the slit.
• There’s a path
light from each part
of the slit to screen
• Central bright
fringe wider than
• Get minima where
the points sum to
zero (dark spots).
all of these interfere
Slit width = a
Comparable in size to λ
Single-Slit Diffraction Pattern
• Intensity on screen depends on direction θ
• Get destructive interference when:
sin θ = mλ/a (m = 1, 2, 3, …)
• This formula locates minima
• Interference only occurs when a ~ λ
Slit Width Compared to Wavelength
• Narrow slit:
a << λ à circular wavefronts, medium bright screen • Medium slit: a ~ λ à single-slit pattern H-9 Slit Width Compared to Wavelength • Wide slit: a >> λ
à dot, because light goes in straight lines
• Application: importance of wavelength à can
pick up radio waves when visible light is
blocked since radio wavelengths are longer
Back up a sec…
• What is light anyway? A photon? A wave?
• Wave-Particle Duality
o Light behaves like a particle when emitted by an
atom or absorbed by photographic film or other
o Light behaves like a wave while traveling from a
source to the place where it is detected.
• What does this have to do with electrons?
De Broglie Wavelengths
• Matter can behave like a wave??!?
• Yup! Wavelength = h/momentum
λ = h/mv
This wavelength is called the de Broglie
wavelength, after a French physicist
De Broglie Wavelengths
Example: What is the de Broglie wavelength of a
ping-pong ball of mass 2 grams after it has been
slammed across the table at a speed of 5 m/s?
λ = h/mv
• The de Broglie hypothesis was unexpectedly
experimentally confirmed in 1927 by two
scientists firing electrons at a nickel crystal
• The regular spacing between
atoms in the crystal acts like
a diffraction grating
Electrons over time and probability
• Both matter and light have both wave and
• The type of question that we ask (or the type of
measurement that we seek to make) determines
the properties that we will see!
• The wave and particle natures of matter and light
are two complementary properties, like two
sides to the same coin
• In what ways do photons act like waves? In what
ways do they act like particles?
• In what ways do electrons act like waves? In
what ways do they act like particles?
• What’s the experimental evidence?
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