Physical Optics Ch 37 and 38

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Physical Optics Ch 37 and 38
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Physical Optics
Light is an electromagnetic wave. Wave
properties Diffraction wave bends around
corners, spreads out from narrow
slits Interference waves from two or more
sources interfere
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Electromagnetic Waves
E
B
Eo
v
Usually we keep track of the electric field E
amplitude
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The Electromagnetic Spectrum
l (m) f (Hz) 300 106
3 108 3 x 10-3 1011 3 x 10-6
1014 7 x 10-7 5x1014 4 x 10-7 3 x 10-9
1017 3 x 10-12 1020
Infrared 780 nm Red Yellow 600
nm Green 550 nm Blue 450 nm Violet 380
nm Ultraviolet
Radio TV Microwave Infrared Visible Ultraviolet
X rays g rays
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Infrared 780 nm Red Yellow 600 nm Green 550
nm Blue 450 nm Violet 380 nm Ultraviolet
l
Visible-Light Spectrum
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The EM Spectrum
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Coherent sources are sources that have a constant
phase difference between them (lasers). In
phase sources are sources that have zero phase
difference between them. Monochromatic sources
have the same wavelength. Plane waves mean the
wavefronts are planes Wavefronts are surfaces on
which we have only crests, or troughs for
example.
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Double Slit(Thomas Young, 1801)
m2 m1 m0 m-1 m-2
?
incident light,plane waves
Two slits, separation d
screen
Result Many bright and dark fringes or lines on
the screen
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The slits act as sources of light in phase. Due
to diffraction, the light spreads out after it
passes through each slit. When the two waves
arrive together at some point P on the screen,
they are out of phase, due to the difference in
the length of the paths. This path difference
varies from place to place on the screen.
P
x1
x2
q
d

screen
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Constructive Interference (bright fringe) ?x
m?, or d sin ? m?, m 0, 1, 2,
Destructive Interference (dark fringe) ?x (m
½)?, or d sin ? (m ½) ?, m 0, 1,
2,
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Example 1
2 slits, 0.20 mm apart red light (l 667 nm)

y
0
3 m
screen
Where are a) the m0,1 bright fringes? b)
m0,1 dark fringes? (give values of y)
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Solution
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Example 2
2 slits, red light (l 690 nm)
y
0
50 cm
screen
If the distance between the first and the third
minima in the diffraction pattern is 3.0mm, what
is the width of the slit?
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Solution
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Quiz
Which of the following would cause the separation
between the fringes on the screen to decrease?
(There may be more than one answer.)

1. Increasing the wavelength
2. Decreasing the wavelength
3. Moving the slits closer together
4. Moving the slits farther apart
5. Moving the screen further from the slits
6. Moving the screen closer to the slits

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Test Discussion