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Chapter 35
Interference
PowerPoint® Lectures for
University Physics, Twelfth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by James Pazun
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Wave fronts from a disturbance
• Think back to our first slide on
wave motion when the father
threw an object into the pool
and the boy watched the ripples
proceed outward from the
disturbance. We can begin our
discussion of interference from
just such a scenario, a coherent
source and the waves from it
that can add (constructively or
destructively).
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A “snapshot”
• The “snapshot” of sinusoidal waves spreading out from
two coherent sources.
• Consider Figure 35.2.
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Q35.1
Two sources S1 and S2 oscillating in phase emit sinusoidal waves.
Point P is 7.3 wavelengths from source S1 and 4.3 wavelengths
from source S2. As a result, at point P there is
A. constructive interference.
B. destructive interference.
C. neither constructive nor destructive interference.
D. not enough information given to decide.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A35.1
Two sources S1 and S2 oscillating in phase emit sinusoidal waves.
Point P is 7.3 wavelengths from source S1 and 4.3 wavelengths
from source S2. As a result, at point P there is
A. constructive interference.
B. destructive interference.
C. neither constructive nor destructive interference.
D. not enough information given to decide.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Q35.2
Two sources S1 and S2 oscillating in phase emit sinusoidal waves.
Point P is 7.3 wavelengths from source S1 and 4.6 wavelengths
from source S2. As a result, at point P there is
A. constructive interference.
B. destructive interference.
C. neither constructive nor destructive interference.
D. not enough information given to decide.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A35.2
Two sources S1 and S2 oscillating in phase emit sinusoidal waves.
Point P is 7.3 wavelengths from source S1 and 4.6 wavelengths
from source S2. As a result, at point P there is
A. constructive interference.
B. destructive interference.
C. neither constructive nor destructive interference.
D. not enough information given to decide.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Two-source interference of light
•
Figure 35.4 shows two waves interfering constructively and
destructively.
•
Young did a similar experiment with light. See below.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Interference from two radio stations
• Radio station operating at 1500 kHz has two antennas spaced
400m apart. In which directions is the intensity greatest in the
resulting radiation pattern far away (>> 400m) from the
antennas?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
As the waves interfere, they produce fringes
• Consider Figure 35.6 below.
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Intensity distribution
• Figure 35.10, below, displays
the intensity distribution from
two identical slits interfering.
• Follow Example 35.3.
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Q35.3
In Young’s experiment, coherent light passing through
two slits (S1 and S2) produces a pattern of dark and
bright areas on a distant screen. If the wavelength of the
light is increased, how does the pattern change?
A. The bright areas move closer together.
B. The bright areas move farther apart.
C. The spacing between bright areas remains the
same, but the color changes.
D. any of the above, depending on circumstances
E. none of the above
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A35.3
In Young’s experiment, coherent light passing through
two slits (S1 and S2) produces a pattern of dark and
bright areas on a distant screen. If the wavelength of the
light is increased, how does the pattern change?
A. The bright areas move closer together.
B. The bright areas move farther apart.
C. The spacing between bright areas remains the
same, but the color changes.
D. any of the above, depending on circumstances
E. none of the above
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Q35.4
In Young’s experiment, coherent light passing through
two slits (S1 and S2) produces a pattern of dark and
bright areas on a distant screen.
What is the difference between the distance from S1 to
the m = +3 bright area and the distance from S2 to the
m = +3 bright area?
A. three wavelengths
B. three half-wavelengths
C. three quarter-wavelengths
D. not enough information given to decide
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A35.4
In Young’s experiment, coherent light passing through
two slits (S1 and S2) produces a pattern of dark and
bright areas on a distant screen.
What is the difference between the distance from S1 to
the m = +3 bright area and the distance from S2 to the
m = +3 bright area?
A. three wavelengths
B. three half-wavelengths
C. three quarter-wavelengths
D. not enough information given to decide
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thin films will interfere
• The reflections of
the two surfaces in
close proximity will
interfere as they
move from the film.
• Figure 35.11 at
right displays an
explanation and a
photograph of thinfilm interference.
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Interference between mechanical and EM waves
• Figure 35.13 compares the interference of mechanical and EM
waves.
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Soap bubble
You want to make a soap bubble that will
primarily reflect red light (700 nm
wavelength in vacuum). How thick
should the bubble be? Index of
refraction of soapy water n = 1.33.
How could you reflect blue light? (no
numbers, just explain)
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An air wedge between two glass plates
•
Just like the thin film, two waves reflect back from the air wedge in
close proximity, interfering as they go.
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Q35.6
An air wedge separates two
glass plates as shown. Light of
wavelength l strikes the upper
plate at normal incidence. At a
point where the air wedge has
thickness t, you will see a
bright fringe if t equals
A. l/2.
B. 3l/4.
C. l.
D. either A. or C.
E. any of A., B., or C.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
A35.6
An air wedge separates two
glass plates as shown. Light of
wavelength l strikes the upper
plate at normal incidence. At a
point where the air wedge has
thickness t, you will see a
bright fringe if t equals
A. l/2.
B. 3l/4.
C. l.
D. either A. or C.
E. any of A., B., or C.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley
Thick films and thin films behave differently
•
Refer to Figure 35.14 in the middle of this slide.
•
Read Problem-Solving Strategy 35.1.
•
Follow Example 35.4, illustrated by Figure 35.15 at the bottom of the
slide.
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Thin-film examples
•
Consider Example 35.5.
•
Consider Example 35.6, illustrated by Figure 35.16 shown
below.
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Newton’s rings
• Figure 35.17 illustrates the interference rings resulting from an
air film under a glass item.
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Using fringes to test quality control
• An optical
flat will only
display
even,
concentric
rings if the
optic is
perfectly
ground.
• Follow
Example
35.7.
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Michelson and Morley’s interferometer
•
In this amazing experiment at Case Western Reserve, Michelson and Morley
suspended their interferometer on a huge slab of sandstone on a pool of
mercury (very stable, easily moved). As they rotated the slab, movement of
the earth could have added in one direction and subtracted in another,
changing interference fringes each time the device was turned a different
direction. They did not change. This was an early proof of the invariance of
the speed of light.
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