Transcript Diffraction and Interference of Light

Diffraction and Interference
of Light
By: Karen Cooper
Diffraction is…
 the bending of waves, esp. sound and light
waves, around obstacles in their path.
Interference is…
 the process in which two or more light,
sound, or electromagnetic waves of the
same frequency combine to reinforce
or cancel each other, the amplitude of
the resulting wave being equal to the
sum of the amplitudes of the
combining waves.
19.1
When Light
Waves
Interfere
Isaac
Newton believed light was made up
of fast moving, tiny particles- he called
these corpuscles. He thought the edges of
shadows weren't perfectly sharp because
of infraction. Newton probably never
thought that the light's wavelengths might
be so tiny that they could produce such
small effects of diffraction.
Diffraction
• The Dutch scientist Christian Huygens
(1629-1695) proposed a wave model
to explain diffraction. These wavelets
expand in every direction and are in
step with one another. Light sources
consist of an infinite number of points,
which generate a plane wave front.
http://demonstrations.wolfram.com/HuygenssPrinciple/
Karen Cooper
English physician Thomas Young
(1773-1829) read Newton's book on
optics: convinced Newton's
description of light behavior in
optics could be explained if light
were a wave with an extremely small
wavelength.
Vocabulary
• Interference fringes(19.1): pattern of dark/light bands
on a screen due to constructive/destructive interference
of light waves passing through two narrow, closely
spaced slits
• Monochromatic light (19.1): light of only one
wavelength
• Coherent waves(19.1): waves that are in phase--crests
and troughs reach the same point together
Young’s twoslit experiment
• Young could measure light's
wavelengths and provided additional
evidence of the waves nature of light.
By directing the beam of light at two
narrow, closely spaced slits in barrier,
light diffracted and the two rays
overlapped. This created light and dark
bands.
• Only a small part of light passes through a
single slit, emitting one wavelength. The
waves are in a phase, crests reaching the
same point at same time, so did troughs.
• Waves spread after passing through slit and
fall on a double slit. Waves diffracted from
double slit act as two sources of new circular
waves spreading out from barrier.
Cheng, Poon S., Mrs. "Chapter 4: Young’s Double Slit Experiment." Singapore A Level Physics Syllabus. N.p., n.d. Web. 6 Feb.
2011.<http://h2physics.org/>.
Where two crests overlap
(waves interfere constructively),
light intensity increases creating
a bright band on screen. Crests
and troughs meet (interfere
destructively) and cancel one
another to create dark region.
Constructive
: positive
growth.
Destructive
: negative
growth.
Constructive Interference. Tipler, Paul A. Physics for Scientists and Engineers. 4th Ed. New York: W.H. Freeman and Company, 1999.
Destructive Interference. Tipler, Paul A. Physics for Scientists and Engineers. 4th Ed. New York: W.H. Freeman and Company, 1999.
Diffraction of
white light
Constructive interference produces a bright
central band and other bands on either side.
Dark areas are where destructive
interference occurs between bright bands.
White light in double slit diffraction causes
colors instead of light/dark bands. Positions
of constructive/destructive bands depends on
light's wavelength.
Red Light
Blue Light
White Light
Karen Cooper
White Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.
Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.
Measuring the
Wavelength of a
Light Wave
No
matter the wavelength used,
light reaching the point Po
travels the same distance from
the slits. Wavelengths interact
constructively. The first bright
band on either side of the
central band are called firstorder line.
Karen Cooper
Equation: Wavelength Using
Double Slit Interference
Distance between P0 and
P1: x
Distance between screen
and slits: L
Separation of two slits: d
Equation: x/L = λ/d
Wavelength Using DoubleSlit Interference: λ = xd/L
Single-Slit
Diffraction
Grimaldi first noted that diffraction
occurs in both light and sound, but since
light's wavelengths are much smaller,
diffraction is less obvious.
 EX: walking near the band room, you can
hear the band before you actually see
them.

Playing Piano. N.d. Piano Tutorial. N.p., 2010. Web. 8 Feb. 2011.,
http://www.pianotutorial.com/articles/piano-in-a-band.html>.
From one to
many slits
• Light passes through a single slit's small
opening. This creates a central band with
dimmer bands on either side.
Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.
• A single slit had a width called w. Imagine the
slit is divided into large number of smaller slits
called dw.
• Choosing a pair of slits so each has the same
separation: divide the slit into two equal parts,
pairs are separated by distance of w/2.
• Any slit in the top half will have another slit in
the bottom with a distance of w/2.
Karen Cooper
Measuring a
wavelength of light
 The slit is illuminated, so a central
bright band appears at Po on
screen.
 The dark band from the central
light band create a situation similar
to the double-slit interference.
 Paths differ by λ/2 and separation
between slits is w/2 instead.
Red Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.
Karen Cooper
White Light. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.2011. <http://www.itp.uniannover.de/~zawischa/ITP/multibeam.html>.
Equation: Ratio of Sides
of a Triangle
• x/L = (λ/2)/(w/2) = λ/w
• Distance between slits and
screen: L
• Distance between Po and P1: x
• Wavelength: λ
• Width of a single slit: w
Equation: Distance Between
Central and 1st Dark Bands
 x = λL/w
 Distance between slits and
screen: L
 Distance between Po and P1: x
 Wavelength: λ
 Width of a single slit: w
19.2
Applications
of Diffraction
 The colors seen on a beetles back are
caused by diffraction. Hard ridges cover
the beetles back and the spaces between
them cause diffraction like slits do. This
produces interference effects. The ridges
and slits enhance the interference pattern
in series. A CD does the same thing,
diffracting light between its grooves.
Susan. Lavender Beetle. N.d. ShopCurious. Syrox eMedia, 31 July 2009. Web. 7
Feb. 2011. http://shopcurious.blogspot.com/2009/07/curiouscolours-of-nature-in-fashion.html>.
Diffraction Gratings
• Diffraction gratings are used to measure
the wavelength of light. Diffraction
gratings are made by scratching very
fine lines on glass using the point of a
diamond. The spaces between the
scratches are like slits.
• EX: jewelry creates spectrum like that
on the surface of a CD.
Diamond. N.d. Colour Museum. N.p., 2009. Web. 7 Feb. 2011.
<http://www.coloure.experience.org/index.htm>.
CD. N.d. Multiple-beam interference. N.p., 2009. Web. 7 Feb.
2011. <http://www.itp.uniannover.de/~zawischa/I
TP/multibeam.html>.
The interference pattern produced
by diffraction grating has bright
bands cause by a double-slit.
Individual colors can be
distinguished more easily with this
and wavelengths can be measured
more precisely in diffraction grating
then double-slits.
Equation: Wavelength Using
a Diffraction Grating
• λ = xd/L = d sin θ
• Distance between P0 and P1: x
• Distance between screen and slits: L
• Separation of two slits: d
• Wavelength: λ

A grating spectroscope is used to
measure light wavelengths. As you
look through, a light falls on the slit
and creates a series of bright bands
on either side of a central band. At the
calibrated base of the spectrometer,
the angle θ is shown. Since d is
known, λ can be calculated.
Ng., H. K., Dr. Diffraction Grating Spectrometer. N.d. N.p., 30 Dec. 1996. Web. 8 Feb. 2011.<http://www.physics.fsu.edu/users/ng/courses/phy2054c/
labs/expt09/expt-09.htm>.
Vocabulary
• Diffraction grating(19.2): a device
with parallel ridges that reflect light
and form an interference pattern.
Resolving Power
of Lenses
A telescopes lens diffracts light just
as a slit does. The pattern is wide if
the lens is small.
EX: a single star will appear spread
out while two close stars may blur
together so it's unknown if there
are two or only one.
Rayleigh-Criteria: Stars. N.d. Wikimedia Commons. N.p., 30 May 2010. Web. 8 Feb. 2011.
<http://commons.wikimedia.org/wiki/File:Rayleigh_fromcalculateddiffractiondisc_edit
edwithgimp.png>.
Lord Rayleigh established that
when the central bright band of one
star falls on the first dark band of
the second, the two stars will be
resolved. By reducing the lens's
size, the effects of diffraction can
be reduced.
 The resolving power of microscopes
is limited by diffraction as well. The
lens can't be enlarged, but the
wavelength of light can be reduced.
 Since blue light forms a narrower
pattern than red light, it is used for
microscopes while red light is used
for telescopes.
Vocabulary
 Rayleigh criterion(19.2): when
the central bright band of one
star falls on the first dark band
of the second, the two stars will
be just resolved.
Works Cited

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Zitzewitz, Paul W. Physics Principles and Problems.
2002 ed. New York: Glencoe, 2002. Print.
Tipler, Paul A. Physics for Scientists and Engineers. 4th
Ed. New York: W.H. Freeman and Company, 1999.
Cheng, Poon S., Mrs. "Chapter 4: Young’s Double Slit
Experiment." Singapore A Level Physics Syllabus.
N.p., n.d. Web. 6 Feb. 2011.<http://h2physics.org/>.
Susan. Lavender Beetle. N.d. ShopCurious. Syrox
eMedia, 31 July 2009. Web. 7 Feb. 2011. http:
//shopcurious.blogspot.com/2009/07/curiouscolours-of-nature-in-fashion.html>.
Diamond. N.d. Colour Museum. N.p., 2009. Web. 7 Feb.
2011. <http://www.coloure.experience.org/
index.htm>.
White Light. N.d. Multiple-beam interference. N.p.,
2009. Web. 7 Feb.2011. <http://www.itp.
uniannover.de/~zawischa/ITP/multibeam.html>.