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					Physics for Scientists and Engineers, 6e

Chapter 40 - Introduction to Quantum Physics
         This figure shows two stars in the constellation Orion.
         Betelgeuse appears to glow red, while Rigel looks
         blue in color. Which star has a higher surface

    1.    Betelgeuse
    2.    Rigel                          25% 25% 25% 25%

    3.    They both have the same
          surface temperature.
    4.    Impossible to determine.

                                          1      2      3     4
1    2    3   4   5
A very hot star will have its peak in the blackbody
intensity distribution curve at wavelengths shorter
than the visible. As a result, more blue light is
emitted than red light.
         While standing outdoors one evening, you are exposed to the
         following four types of electromagnetic radiation: yellow light
         from a sodium street lamp, radio waves from an AM radio
         station, radio waves from an FM radio station, and microwaves
         from an antenna of a communications system. Rank these
         types of waves in terms of increasing photon energy, lowest
                                              25% 25% 25% 25%
    1.    sodium light, AM, FM,
    2.    AM, FM, sodium light,
    3.    AM, FM, microwave, sodium
    4.    microwave, sodium light, AM,

                                               1       2       3      4
1    2    3   4   5
The order of photon energy will be the
same as the order of frequency. See Figure
34.12 for a pictorial representation of
electromagnetic radiation in order of
         Consider one of the curves shown in this figure.
         Suppose the intensity of the incident light is held
         fixed but its frequency is increased. The stopping
         potential in the figure:

    1.   remains fixed
    2.   moves to the right               33%     33%     33%
    3.   moves to the left

                                           1        2          3
1    2   3   4   5
When the frequency is increased, the photons
each carry more energy, so a stopping potential
larger in magnitude is required for the current to
fall to zero.
         Note that for any given scattering angle θ, Equation 40.11
         (seen below) gives the same value for the Compton shift for
         any wavelength. Keeping this in mind, for which of the following
         types of radiation is the fractional shift in wavelength at a given
         scattering angle the largest?

    1.    radio waves                           25% 25% 25% 25%
    2.    microwaves
    3.    visible light
    4.    x-rays

                                                  1       2       3       4
1    2    3   4    5
The shift Δλ is independent of λ. Thus, the
largest fractional shift will correspond to the
smallest wavelength.
         We have discussed two wavelengths associated
         with the electron—the Compton wavelength and the
         de Broglie wavelength. Which is an actual physical
         wavelength associated with the electron?
    1.    the Compton wavelength
    2.    the de Broglie wavelength    25% 25% 25% 25%

    3.    both wavelengths
    4.    neither wavelength

                                        1     2     3     4
1    2    3   4   5
The Compton wavelength (Section
40.3) is a combination of constants and
has no relation to the motion of the
electron. The de Broglie wavelength
(Eq. 40.15) is associated with the
motion of the electron through its
         As an analogy to wave packets, consider an “automobile
         packet” that occurs near the scene of an accident on a
         freeway. The phase speed is analogous to the speed of
         individual automobiles as they move through the backup
         caused by the accident. The group speed can be identified as
         the speed of the leading edge of the packet of cars. For the
         automobile packet,

                                              33%      33%      33%
    1.   the group speed is the same as
         the phase speed.
    2.   the group speed is less than
         the phase speed.
    3.   the group speed is greater than
         the phase speed.

                                               1         2         3
1    2   3   4   5
The group speed is zero because the leading
edge of the packet remains fixed at the location
of the accident.
         As another analogy to wave packets, consider a “runner
         packet” that occurs at the start of a footrace of length L. As the
         runners begin the race, the packet of runners spreads in size
         as the faster runners outpace the slower runners. The phase
         speed is the speed of a single runner, while we can identify the
         group speed vg as the speed with which the average position
         of the entire packet of runners moves. The time interval for the
         winning runner to run the race is
                                                 33%      33%       33%

    1.   greater than L/vg.
    2.   equal to L/vg.
    3.   less than L/vg.

                                                  1          2         3
1    2   3   4   5
The phase speed of the winning runner is
larger than the average speed of all the
runners, so the time interval for the winning
runner is less than L/vg.
         The location of a particle is measured and specified
         as being exactly at x = 0, with zero uncertainty in
         the x direction. How does this affect the uncertainty
         of its velocity component in the y direction?
    1.   It does not affect it.
    2.   It makes it infinite.           33%     33%     33%
    3.   It makes it zero.

                                          1        2        3
1    2   3   4   5
The uncertainty principle relates uncertainty in
position and velocity along the same axis. The
zero uncertainty in position along the x axis
results in infinite uncertainty in its velocity
component in the x direction, but it is
unrelated to the y direction.
         A quantum argument for the phenomenon of diffraction of light claims that
         photons passing through a narrow slit have been localized to the width of the
         slit. Because we have gained information about their position, they must have a
         larger uncertainty in momentum along the plane of the screen in which the slit
         is cut. Thus, the photons gain momentum perpendicular to their original
         direction of propagation and spread out, forming on a screen a bright area that
         is wider than the slit. Suppose we are observing diffraction of light and
         suddenly Planck’s constant drops to half its previous value. This quantum
         argument for diffraction would claim that

                                                          33%        33%         33%

    1.     the bright area on the
           screen is unchanged.
    2.     the bright area on the
           screen becomes wider.
    3.     the bright area on the
           screen becomes narrower.

                                                           1            2            3
1    2     3    4    5
According to the uncertainty principle, if Planck’s
constant is smaller, the uncertainty in momentum
can be smaller and the momentum perpendicular
to the original direction of propagation would be
smaller. Note that classical wave theory does not
include Planck’s constant, so it would predict no

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