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Chapter 36
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Diffraction (cont.)
The diffraction grating
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A diffraction grating is an
array of a large number of slits
having the same width and
equal spacing. (See Figure
36.16 at the right.)
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Grating spectrographs
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A diffraction grating can be used to disperse light into a spectrum.
The greater the number of slits, the better the resolution.
Figure 36.18(a) below shows our sun in visible light, and in (b)
dispersed into a spectrum by a diffraction grating.
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Q36.5
Consider two diffraction gratings. One grating has 100
lines/mm, and the other one has 200 lines/mm. Both
gratings are illuminated with a beam of the same
monochromatic light. Which grating produces the
greater dispersion?
A. The dispersion cannot be determined without additional
information
B. The grating with 100 lines/mm produces the greater
dispersion.
C. Both gratings produce the same dispersion.
D. The grating with 200 lines/mm produces the greater
dispersion.
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A36.5
Consider two diffraction gratings. One grating has 100
lines/mm, and the other one has 200 lines/mm. Both
gratings are illuminated with a beam of the same
monochromatic light. Which grating produces the
greater dispersion?
A. The dispersion cannot be determined without additional
information
B. The grating with 100 lines/mm produces the greater
dispersion.
C. Both gratings produce the same dispersion.
D. The grating with 200 lines/mm produces the greater
dispersion.
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X-ray diffraction
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When x rays pass through a crystal, the crystal behaves like
a diffraction grating, causing x-ray diffraction. Figure 36.20
below illustrates this phenomenon.
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A simple model of x-ray diffraction
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Constructive interference is essentially the reason the
angle of reflection equals the angle of incidence.
The Bragg condition for constructive interference is
2d sin = m.
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Circular apertures
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An aperture of any shape forms a diffraction pattern.
Figures 36.25 and 36.26 below illustrate diffraction by a circular
aperture. The airy disk is the central bright spot.
The first dark ring occurs at an angle given by sin1 = 1.22 /D.
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Diffraction and image formation
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Diffraction limits the
resolution of optical
equipment, such as
telescopes.
The larger the aperture, the
better the resolution. Figure
36.27 (right) illustrates this
effect.
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Bigger telescope, better resolution
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Because of diffraction, large-diameter telescopes, such as
the VLA radio telescope below, give sharper images than
small ones.
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Q36.6
You use a telescope lens to form an image of two
closely spaced, distant stars. Which of the following
will increase the resolving power?
A. Use a filter so that only the blue light from the stars
enters the lens.
B. Use a filter so that only the red light from the stars
enters the lens.
C. Use a lens of smaller diameter.
D. more than one of the above
© 2012 Pearson Education, Inc.
A36.6
You use a telescope lens to form an image of two
closely spaced, distant stars. Which of the following
will increase the resolving power?
A. Use a filter so that only the blue light from the stars
enters the lens.
B. Use a filter so that only the red light from the stars
enters the lens.
C. Use a lens of smaller diameter.
D. more than one of the above
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What is holography?
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By using a beam splitter and mirrors, coherent laser light
illuminates an object from different perspectives.
Interference effects provide the depth that makes a threedimensional image from two-dimensional views. Figure
36.28 below illustrates this process.
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How does holography work?
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Initially an interference pattern is created in film, when light
shines from behind on that film, it recreates the original image.
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An example of holography
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Figure 36.32 below shows photographs of a holographic
image from two different angles, showing the changing
perspective.
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