Download Diffraction and Interference - Polson 7-8

Chapter 16:
Interference
and Diffraction
Objectives
• Understand “Huygen’s Principle.”
• Understand how waves diffract.
• Explain how wavelength affects diffraction.
Huygen’s Principle
Christian Huygens: wavelets
produce a wavefront.
Huygen’s Principle and Diffraction
diffraction: the bending of
a wave as it passes around
an obstruction or through
an opening.
Huygen’s principle
helps to show how
waves diffract.
Huygen’s Principle also
shows reflection/refraction
Diffraction and Ocean Waves
straight wavefront
wavefront diffracts around
the edge of a jetty
Diffraction and
Wavelength
• shorter waves
diffract less,
forming shadows
• longer waves
diffract more,
virtually no
shadow
shadows
Diffraction and Wavelength
Short light waves make shadows, but
longer sound waves do not.
Electron Microscopes
• light diffracts
around very
small objects
• electron
microscopes
make smaller
waves that
reflect
Objectives
• Understand how interference fringes are
formed by a double-slit or a diffraction
grating.
• Be able to calculate the wavelength of light
based on an interference pattern.
Interference Fringes
Thomas Young “proved” that light is
made of waves with his famous
“double slit” experiment.
“Double Slit” Diffraction Math
first order
bright
Interference can be
used to calculate l :
90-q
n
sinq = (l / d)
l = d·sinq
q
d
screen
q = tan-1 (y / L)
d
l
l
q
central
bright
Diffraction Grating
n
Each successive slit
in a diffraction grating
diffracts light to form
a fringe.
n+1
n+2
n+3
l = d·sinq can be
used to calculate
wavelength
Calculating Wavelength
What is the wavelength of a laser if fringes
separated by 35.9 cm are made on a screen
98.2 cm from a diffraction grating with lines
spaced 0.000200 cm apart?
Objectives
• Understand the importance of Thomas
Young’s double-slit experiment.
• Understand how interference patterns can be
used to determine crystalline and molecular
structures.
• Understand how single-slit diffraction
patterns form.
Thomas Young, Superstar
• He read by age 2 and by age 4 had read the
Bible twice; he also played several instruments.
• He spoke eight languages by age 14.
• As a physicist, he helped define the concept of
energy, he studied the elastic properties of
materials, and he explained how waves
constructively and destructively interfere.
• He was a practicing physician.
• Based on his studies of the eye, he determined
how the eye focuses and he helped develop the
idea of color addition.
• He was the first person to successfully use the
Rosetta Stone to decipher Egyptian
hieroglyphics!
1773 - 1829
Young’s Interference Experiment
• Thomas Young used a doubleslit to “prove” that light is
made of waves in 1802.
• Simon Poisson:
monochromatic light should
make a bright spot in the
center of a shadow.
?
Diffraction and Crystal Structure
Crystal lattice
structures are
determined by
observing patterns
formed by
diffracting X-rays.
Rosalind Franklin’s photo shows
the X-ray diffraction pattern made
by DNA. Watson and Crick stole
the photo and determined the
double-helix structure.
Single Slit Diffraction
With a single slit, dark bands are
observed where destructive
interference occurs.
½l/½d=½w/L
l / d = w / 2L
w = 2L l /d
dark
n
n+½
½d
½w
L
½l
dark
Resolving Power
resolving power: the
ability to see two images
that are close together
Airy disk
Objectives
• Understand how iridescence occurs due to
thin-films and other microscopic structures.
• Understand how lasers work.
Iridescence
iridescence: the interference of
colors caused by reflection and
refraction in thin films
Iridescence
incident rays
in-phase
• Rays of a single color reflect off
two thin layers and cancel.
• Occurs if the thickness is an odd
multiple of ¼ l (¼ , ¾, etc.).
• If red cancels, it appears cyan
because W – R = C.
• Different thicknesses result in
different colors.
reflecting rays
out-of-phase
Iridescence
blue cancels,
yellow observed
CDs and DVDs have pitted
layers that produce
iridescence.
¼l
red cancels,
cyan observed
¼l
Laser Light
laser: light amplification by the stimulated
emission of radiation
Typical Light
Laser Light
• incoherent (out-of-phase)
• coherent (all waves in-phase)
• polychromatic (many l)
• monochromatic (single l)
• very intense
• narrow beam
• somewhat polarized
Stimulated Emission
• An excited atom usually
emits a photon
spontaneously.
• Einstein suggested that a
passing photon of proper
energy can stimulate an
excited atom to emit a
photon. The two photons
will be coherent.
Population Inversion
• Einstein said that if a majority of atoms were excited
(a population inversion), a group of in-phase photons
would be produced through stimulated emissions
• He imagined the first laser.
How a Helium-Neon Laser Works
• High voltage excites He atoms.
• He atoms collide with Ne atoms, transfer energy, and
produce a Ne population inversion.
• Ne atoms undergo stimulated emissions and
produce laser light.
• The gases are in a long glass tube with slightly
concave, mirrored ends. The laser light reflects back
and forth, increasing in intensity.
• One end of the tube allows 1% of the light to
escape—the laser beam.
First Ruby Laser: 1960