Electromagnetism & Light
Interference & Diffraction
Interference
and Diffraction
33-1 Phase Difference and Coherence
33-2 Interference in Thin Films
33-3 Two-Slit Interference Pa tt er n
33-4 Diffraction Pattern of a Single Slit
33-5 Using Phasors to Add Harmonic Waves
33-6 Fraunhofer and Fresnel Diffraction
33-7 Diffraction and Resolution
33-8 Diffraction Gratings
Interference and diffraction are the important phenomena that distinguish
waves from particles.* Interference is the formation of a lasting intensity pat-
tern by two or more waves that superpose in space. Diffraction is the bending
of waves around corners that occurs when a portion of a wavefront is cut off
by a barrier or obstacle.
In this chapter, we will see how the pattern of the resulting wave can be cal-
culated by treating each point on the original wavefront as a point source,
according to Huygens’s principle, and calculating the interference pattern
resulting from these sources.
33
CHAPTER
Have you ever wondered
if the phenomenon that produces
the bands that you see in
the light reflected off a soap bubble
has any practical applications?
(See Example 33-2.)
?
1141
WHITE LIGHT IS REFLECTED OFF A SOAP
BUBBLE. WHEN LIGHT OF ONE
WAVELE NGTH I S INCI DENT O N A T HIN
SOAP-AND-WATER FILM, LIGHT IS
REFLECTED FROM BOTH THE FRONT AND
THE BACK SURFACES OF THE FILM.
IFTHE ORDER OF MAGNITUDE OF THE
THICKNESS OF THE FILM IS ONE
WAVELE NGTH O F TH E LIGH T, THE TWO
REFLECTED LIGHT WAVES INTERFERE.
IFTHE TWO REFLECTED WAVES ARE
OUT OF PHASE, THE REFLECTED WAVES
INTERFERE DESTRUCTIVELY, SO THE NET
RESULT IS THAT NO LIGHT IS
REFLECTED. IF WHITE LIGHT, WHICH
CONTAINS A CONTINUUM OF
WAVELE NGTHS, IS IN CIDENT ON THE
THIN FILM, THEN THE REFLECTED WAVES
WILL INTERFERE DESTRUCTIVELY
ONLY FOR CERTAIN WAVELENGTHS,
AND FOR OTHER WAVELENGTHS THEY
WILL INTERFERE CONSTRUCTIVELY.
THIS PROCESS PRODUCES THE
COLORED FRINGES THAT YOU SEE IN
THE SOAP BUBBLE. (Aaron Haupt/
Photo Researchers.)
180°
*Before you study this chapter, you may wish to review Chapter 15 and Chapter 16, where the general topics of inter-
ference and diffraction of waves are first discussed.
*
*
Diffraction - Single Slit
Neil Alberding (SFU Physics) Physics 121: Optics, Electricity & Magnetism Spring 2010 3 / 1
Name ______________________ Date(YY/MM/DD) ______/_________/_______
St.No. __ __ __ __ __-__ __ __ __ Section_________Group #________
UNIT 31: INTERFERENCE AND DIFFRACTION
Interference of two circular waves, snapshots of absolute value of
(real,scalar) wave field for different wave lengths and distances of point
sources. (Wikimedia Commons)
OBJECTIVES
1. Understand the creation of double-slit interference and single-slit
diffraction patterns.
2. Measure slit separation using double-slit interference of He-Ne
laser light.
3. Compare single slit diffraction patterns to double-slit patterns.
4. Examine some 2-D diffraction patterns.
5. Construct and calibrate a portable diffraction-grating spectrometer
and use it to examine the spectra of several light sources.
© 2008 by S. Johnson. Adapted from PHYS 131 Optics lab #4
