Chapter 24

of 42 /42
24.1. Model: Balmer’s formula predicts a series of spectral lines in the hydrogen spectrum. Solve: Substituting into the formula for the Balmer series, λ = 91.18 nm 1 2 1 2 2 n = = λ 91.18 nm 410.3 nm 1 2 1 6 2 2 where n = 3, 4, 5, 6, … and where we have used n = 6. Likewise for n = 8 and n = 10, λ = 389.0 nm and λ = 379.9 nm .

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where n = 3, 4, 5, 6, … and where we have used n = 6. Likewise for n = 8 and n = 10, λ =389.0 nm and λ =379.9 nm . 1 6 24.1. Model: Balmer’s formula predicts a series of spectral lines in the hydrogen spectrum. 2 2 2 2 1 n As , Thus, 91.18 nm 364.7 nm n n →∞ → n = 3, 4, 5, 6, … 24.2. Model: Balmer’s formula predicts a series of spectral lines in the hydrogen spectrum. 2 2 1 n →∞ 2 n 1 4 1 02344 8 24.3. Model: Balmer’s formula predicts a series of spectral lines in the hydrogen spectrum. 2 2 1 n 2

Transcript of Chapter 24

  • 24.1. Model: Balmers formula predicts a series of spectral lines in the hydrogen spectrum.Solve: Substituting into the formula for the Balmer series,

    =

    91.18 nm

    12

    12 2n

    =

    = 91.18 nm 410.3 nm12

    162 2

    where n = 3, 4, 5, 6, and where we have used n = 6. Likewise for n = 8 and n = 10, = 389.0 nm and = 379.9 nm .

  • 24.2. Model: Balmers formula predicts a series of spectral lines in the hydrogen spectrum.Solve: Balmers formula is

    =

    91.18 nm

    12

    12 2n

    n = 3, 4, 5, 6,

    As , Thus, 91.18 nm 364.7 nmn n n = ( ) =1 0 42 . .

  • 24.3. Model: Balmers formula predicts a series of spectral lines in the hydrogen spectrum.Solve: Using Balmers formula,

    = =

    = =389.0 nm 91.18 nm1

    21

    14

    1 0 2344 82 2

    2

    n

    nn.

  • 24.4. Model: The angles of incidence for which diffraction from parallel planes occurs satisfy the Braggcondition.Solve: The Bragg condition is 2d mmcos = , where m = 1, 2, 3, For first and second order diffraction,

    2 11d cos = ( ) 2 22d cos = ( )Dividing these two equations,

    cos

    coscos cos cos cos .

    21

    21

    112 2 2 68 41 5= = ( ) = ( ) =

  • 24.5. Model: The angles of incidence for which diffraction from parallel planes occurs satisfy the Braggcondition.Solve: The Bragg condition is 2d mmcos = . For m = 1 and for two different wavelengths,

    2 11 1d cos = ( ) 2 11 1d cos = ( ) Dividing these two equations,

    cos

    cos

    cos

    coscos . .

    =

    = = ( ) =

    11

    1

    1

    11

    1

    540 4408 63 80.15 nm

    0.20 nm

  • 24.6. Model: The angles corresponding to the various orders of diffraction satisfy the Bragg condition.Solve: The Bragg condition for m = 1 and m = 2 gives

    2 1 2 21 2d dcos cos = ( ) = ( )Dividing these two equations,

    coscos cos

    coscos

    . 1 2 1 1245

    245

    269 3= = = =

  • 24.7. Model: The angles corresponding to the various diffraction orders satisfy the Bragg condition.Solve: The Bragg condition is 2d mmcos = , where m = 1, 2, 3, The maximum possible value of m is thenumber of possible diffraction orders. The maximum value of cosm is 1. Thus,

    2 2 2 4 2d m m d= = = ( )( ) = 0.180 nm

    0.085 nm.

    We can observe up to the fourth diffraction order.

  • 24.8. Model: Use the photon model of light.Solve: The energy of the photon is

    E hf h cphoton Js m / s

    m J= = = ( )

    = 6 63 10

    3 0 10500 10

    3 98 10348

    919

    .

    .

    .

    Assess: The energy of a single photon in the visible light region is extremely small.

  • 24.9. Model: Use the photon model of light.Solve: The energy of the x-ray photon is

    E hf h c= = = ( )

    = 6 63 10

    3 0 101 0 10

    1 99 10348

    916

    .

    .

    .

    . Js m / s m

    J

    Assess: This is a very small amount of energy, but it is larger than the energy of a photon in the visiblewavelength region.

  • 24.10. Model: Use the photon model of light.Solve: The energy of a photon with wavelength 1 is E hf hc1 1 1= = . Similarly, E hc2 2= . Since E2 be equalto 2E1,

    hc hc 2 1

    2= 2 12= = =600 nm

    2300 nm

    Assess: A photon with = 300 nm has twice the energy of a photon with = 600 nm. This is an expected result,because energy is inversely proportional to the wavelength.

  • 24.11. Model: Use the photon model of light.Solve: The energy of the single photon is

    E hf h cphoton Js m / s

    m J= = =

    ( ) ( )

    =

    6 63 10 3 0 10

    1 0 101 99 10

    34 8

    619. .

    .

    .

    = = ( ) ( ) = E N Emol A photon J J6 023 10 1 99 10 1 2 1023 19 5. . .Assess: Although the energy of a single photon is very small, a mole of photons has a significant amount of energy.

  • 24.12. Solve: Your mass is, say, m 70 kg and your velocity is 1 m/s. Thus, your momentum is p = mv (70 kg)(1 m/s) = 70 kg m/s. Your de Broglie wavelength is

    = =

    hp

    6 63 10 9 1034

    36. Js70 kg m / s

    m

  • 24.13. Solve: (a) For an electron, the momentum p mv v= = ( )9 11 10 31. kg . The de Broglie wavelength is = = h

    p0 20 10 9. m =

    ( ) =

    0 20 109 11 10

    3 6 10934

    316

    .

    .

    . m 6.63 10 Js

    kg m / s

    vv

    (b) For a proton, p mv v= = ( )1 67 10 9. kg . The de Broglie wavelength is = = =

    ( ) =

    hp v

    v0 20 10 0 20 10 6 63 101 67 10

    2 0 109 934

    273

    . .

    .

    .

    . m m Js

    kg m / s

  • 24.14. Solve: The kinetic energy of a particle of mass m is related to its momentum by E p m= 2 2 . Using = h p , for an electron we have

    E hm

    = =

    ( )( ) ( ) =

    2

    2

    34 2

    31 9 219

    26 63 10

    2 9 11 10 1 0 102 4 10

    .

    . .

    .

    Js

    kg m J

  • 24.15. Solve: (a) The baseballs momentum is p = mv = (0.200 kg)(30 m/s) = 6.0 kg m/s. The baseballs deBroglie wavelength is

    = = =

    hp

    6 63 10 1 1 1034

    34..

    Js6.0 kg m / s

    m

    (b) Using = =h p h mv , we have

    vh

    m= =

    ( ) ( ) =

    6 63 10

    0 20 101 7 10

    34

    923.

    .

    .

    Js0.200 kg m

    m / s

  • 24.16. Model: The momentum of a wave-like particle has discrete values given by p n h Ln = ( )2 where n =1, 2, 3, .Solve: Because we want the smallest box and the momentum of the electron can not exceed a given value, n mustbe minimum. Thus,

    p mv hL

    L hmv

    1

    34

    312 26 63 10

    2 9 11 100 036= = = =

    ( )( ) =

    .

    .

    .

    Js kg 10 m / s

    mm

  • 24.17. Model: A confined particle can have only discrete values of energy.Solve: From Equation 24.14, the energy of a confined electron is

    E hmL

    nn =2

    22

    8 n = 1, 2, 3, 4,

    The minimum energy is

    E hmL

    L hmE1

    2

    21

    34

    31 18

    10

    8 86 63 10

    8 9 11 10 1 5 102 0 10= = =

    ( ) ( ) =

    .

    . .

    .

    Js

    kg J m = 0.20 nm

  • 24.18. Model: Model the 5.0-fm-diameter nucleus as a box of length L = 5.0 fm.Solve: The protons energy is restricted to the discrete values

    E hmL

    nn

    nn = =( )

    ( ) ( ) = ( )

    2

    22

    34 2 2

    27 15 212 2

    86 63 10

    8 1 67 10 5 0 101 316 10

    .

    . .

    .

    Js

    kg m J

    where n = 1, 2, 3, For n = 1, E1121 316 10= . J , for n = 2, E2

    12 121 316 10 4 5 26 10= ( ) = . . J J , and for n =3, E E3 1

    119 1 18 10= = . J .

  • 24.19. Model: The generalized formula of Balmer predicts a series of spectral lines in the hydrogen spectrum.Solve: (a) The generalized formula of Balmer

    =

    91.18 m1 1

    2 2m n

    with m = 1 and n > 1 accounts for a series of spectral lines. This series is called the Lyman series and the first twomembers are

    12

    2

    2

    91 18

    1 12

    1 13

    =

    = =

    =

    . m 121.6 nm 91.18 nm 102.6 nm

    For n = 4 and n = 5, 3 4 95 0= =97.3 nm and nm. .(b) The Lyman series converges when n . This means 1 02n and 91.18 nm.(c) For a diffraction grating, the condition for bright (constructive interference) fringes is d ppsin = , where p =1, 2, 3, For first-order diffraction, this equation simplifies to d sin = . For the first and second members ofthe Lyman series, the above condition is d dsin sin 1 1 2 2= = = =121.6 nm and 102.6 nm . Dividing these twoequations yields

    sin sin . sin 2 1 10 84375= = ( )

    102.6 nm121.6 nm

    The distance from the center to the first maximum is y L= tan . Thus,

    tan . sin . sin . . 1 1 1 2 214 072 0 84375 14 072 11 84= = = = ( ) ( ) = yL0.376 m1.5 m

    Applying the position formula once again,y L2 2 11 84= = ( ) ( ) = =tan tan . 1.5 m 0.314 m 31.4 cm

  • 24.20. Model: The generalized formula of Balmer predicts a series of spectral lines in the hydrogen spectrum.Solve: (a) The generalized formula of Balmer

    =

    91.18 m1 1

    2 2m n

    with m = 3, and n > 3 accounts for a series of spectral lines. This series is called the Paschen series and thewavelengths are

    =

    =

    91.18 nm 820.62 213

    1 92 2

    2

    n

    n

    n

    The first four members are 1 2 3 4= = = =1876 nm, 1282 nm, 1094 nm, and 1005 nm.(b) The Paschen series converges when n . This means

    1 02 13

    2n

    ( ) =91.18 nm 821 nm

    (c) For a diffraction grating, the condition for bright (constructive interference) fringes is d ppsin = , where p =1, 2, 3, For first-order diffraction, this equation simplifies to d sin = . For the first and second members ofthe Paschen series, the condition is d sin 1 1= and d sin 2 2= . Dividing these two equations yields

    sin sin sin . sin 2 12

    11 10 6834=

    =

    = ( )1282 nm1876 nm

    .

    The distance from the center to the first maximum is y L= tan . Thus,

    tan .1 1 0 4047= = =yL

    0.607 m1.5 m

    1 22 03= . sin . sin . . 2 20 6834 22 03 14 85= ( ) = Applying the position formula once again,

    y L2 2 14 85= = ( ) = =tan tan . 1.5 m 0.398 m 39.8 cm

  • 24.21. Model: Use the photon model of light.Solve: (a) The wavelength is calculated as follows:

    E hf h cgamma Js m / s

    J m= =

    =( ) ( )

    =

    6 63 10 3 0 10

    1 0 102 0 10

    34 8

    1312. .

    .

    .

    (b) The energy of a visible-light photon of wavelength 500 nm is

    E h cvisible Js m / s

    m J= =

    ( ) ( )

    =

    6 63 10 3 0 10

    500 103 978 10

    34 8

    919. .

    .

    The number of photons n such that E nEgamma visible= is

    nEE

    = =

    =

    gamma

    visible

    J J

    1 0 103 978 10

    2 51 1013

    195.

    .

    .

  • 24.22. Model: Use the photon model.Solve: The energy of a 1000 kHz photon is

    E hfphoton Js Hz J= = ( ) ( ) = 6 63 10 1000 10 6 63 1034 3 28. .The energy transmitted each second is 20 103 J . The number of photons transmitted each second is 20 103 J /6 63 10 3 02 1028 31. . = J .

  • 24.23. Model: Use the photon model for the laser light.Solve: (a) The energy is

    E hf h cphoton Js m / s

    m J= = = ( )

    = 6 63 10

    3 10633 10

    3 14 10348

    919

    . .

    (b) The energy emitted each second is 1 0 10 3. J . The number of photons emitted each second is 1 0 10 3. J /3 14 10 19. J = 3 19 1015. .

  • 24.24. Model: Use the photon model for the incandescent light.Solve: The photons travel in all directions. At a distance of r from the light bulb, the photons spread over a sphereof surface area 4r2. The number of photons per second per unit area at the location of your retina is

    3 104 10 10

    2 387 1018 1

    3 29 1 2

    ( ) =

    s

    m s m

    .

    The number of photons that enter your pupil per second is2 387 10 3 0 10 6 75 109 1 2 3 2 4 1. . . ( ) = s m m s

  • 24.25. Model: Use the photon model of light and the Bragg condition for diffraction.Solve: The Bragg condition for the reflection of x-rays from a crystal is 2d mmcos = . To determine the anglesof incidence m, we need to first calculate . The wavelength is related to the photons energy as E hc= . Thus,

    = =( ) ( )

    =

    hcE

    6 63 10 3 0 101 50 10

    1 326 1034 8

    1510. .

    .

    .

    Js m / s J

    m

    From the Bragg condition,

    mm

    dm

    m= =

    ( )( )

    = ( ) = ( ) =

    cos cos.

    .

    cos . cos . .1 110

    91

    11

    21 326 102 0 21 10

    0 3157 0 3157 71 6 m

    m

    Likewise, 2 1 0 3157 2 50 8= ( ) = cos . . and 3 18 7= . . Note that 4 1 0 3157 4= ( )cos . is not allowed becausethe cos cannot be larger than 1. Thus, the x-rays will be diffracted at angles of incidence equal to 18.7, 50.8, and71.6.

  • 24.26. Model: The angles for which diffraction from parallel planes occurs satisfy the Bragg condition.Solve: We cannot assume that these are the first and second order diffractions. The Bragg condition is2d mmcos = . We have

    2 45 6 2 21 0 1d m d mcos . cos . = = +( ) Notice that m decreases as m increases, so 21.6 corresponds to the larger value of m. Dividing these two equations,

    cos .

    cos ..

    45 621 0 1

    0 7494 3

    =

    += =

    m

    mm

    Thus these are the third and fourth order diffractions. Substituting into the Bragg condition,

    d =

    = =

    3 0 070 102 45 6

    1 50 109

    10.

    cos ..

    m m 0.150 nm

  • 24.27. Model: The x-ray diffraction angles satisfy the Bragg condition.Solve: (a) The Bragg condition ( 2d mmcos = ) for normal incidence, m = 0, simplifies to 2d = m. For a thinfilm of a material on a substrate where nair < nmaterial < nsubstrate, constructive interference between the two reflectedwaves occurs when 2d = m, where is the wavelength inside the material.(b) From a thin film with a period of 1.2 nm, that is, with d = 1.2 nm, the two longest x-ray wavelengths that willreflect at normal incidence are

    121

    =

    d 2

    22

    =

    d

    This means that 1 2= ( ) =1.2 nm 2.4 nm and 2 = 1.2 nm .

  • 24.28. Solve: A small fraction of the light wave of an appropriate wavelength is reflected from each littlebump in the refractive index. These little bumps act like the atomic planes in a crystal. The light will be stronglyreflected (and hence blocked in transmission) if it satisfies the Bragg condition at normal incidence ( = 0).

    2d m mn

    = = glassglass

    = =( )( )

    =

    22 0 45 10 1 50

    11 35

    6dnm

    glass m. .. m

  • 24.29. Model: Particles have a de Broglie wavelength given by = h p . The wave nature of the particlescauses an interference pattern in a double-slit apparatus.Solve: (a) Since the speed of the neutron and electron are the same, the neutrons momentum is

    p m v mm

    m vm

    mm v

    m

    mpn n n n

    e

    e nn

    e

    e en

    e

    e= = = =

    where mn and me are the neutrons and electrons masses. The de Broglie wavelength for the neutron is

    nn e

    e

    n

    ee

    n

    = = =

    hp

    hp

    pp

    m

    m

    From Section 22.2 on double-slit interference, the fringe spacing is y L d= / . Thus, the fringe spacing for theelectron and neutron are related by

    y y mm

    yn ne

    ee

    n

    e

    kg kg

    m m 0.818 m= = =

    ( ) = =

    9 11 101 67 10

    1 5 10 8 18 1031

    273 7.

    .

    . .

    (b) If the fringe spacing has to be the same for the neutrons and the electrons,

    y y hm v

    hm v

    v vm

    me n e n

    e e n n

    n ee

    n

    m / s kg kg

    m / s= = = = = ( )

    =

    2 0 10 9 11 101 67 10

    1 09 10631

    273

    .

    .

    .

    .

  • 24.30. Model: Electrons have a de Broglie wavelength given by = h p . The wave nature of the electronscauses a diffraction pattern.Solve: The width of the central maximum of a single-slit diffraction pattern is given by Equation 22.22:

    wL

    a

    Lhap

    Lhamv

    = = = =

    ( ) ( )( ) ( ) ( ) = =

    2 2 2 2 6 63 101 0 10 9 11 10 1 5 10

    9 70 1034

    6 31 64 1.0 m Js

    m kg m / s m 0.970 mm

    .

    . . .

    .

  • 24.31. Model: Neutrons have a de Broglie wavelength given by = h p . The wave nature of the neutronscauses a double-slit interference pattern.Solve: Measurements show that the spacing between the m = 1 and m = 1 peaks is 1.4 times as long as the lengthof the reference bar, which gives the real fringe separation y = 70 m. Similarly, the spacing between the m = 2and m = 2 is 2.8 times as long as the length of the reference bar and yields y = 70 m.

    The fringe separation in a double-slit experiment is y L d= . Hence,

    = = = = =( )( )

    ( ) ( ) ( ) =

    y dL

    hp

    hmv

    y dL

    vhLy md

    6 63 1070 10 1 67 10 0 10 10

    34

    6 27 3

    .

    . .

    Js 3.0 m m kg m

    170 m / s

  • 24.32. Model: Electrons have a de Broglie wavelength given by = h p .Visualize: Please refer to Figure 24.11. Notice that a scattering angle = 60 corresponds to an angle ofincidence = 30 .Solve: Equation 24.6 describes the Davisson-Germer experiment: D mmsin 2 ( ) = . Assuming m = 1, this equationsimplifies to Dsin2 = . Using = h mv , we have

    D hmv

    = =

    ( ) ( ) ( ) = =

    sin.

    . . sin.

    26 63 10

    9 11 10 4 30 10 601 95 10

    34

    31 610

    Js

    kg m / s m 0.195 nm

  • 24.33. Model: A confined particle can have only discrete values of energy.Solve: (a) Equation 24.14 simplifies to

    E hmL

    n nn = =( )

    ( ) ( ) = ( )

    2

    22

    34 2

    31 9 219 2

    86 63 10

    8 9 11 10 0 70 101 231 10

    .

    . .

    .

    Js

    kg m J

    Thus, E119 2 191 231 10 1 1 23 10= ( )( ) = . .J J , E2 19 2 191 231 10 2 4 92 10= ( )( ) = . .J J , and E3 181 11 10= . J.

    (b) The energy is E E2 1 19 19 194 92 10 1 23 10 3 69 10 = = . . . J J J .(c) Because energy is conserved, the photon will carry an energy of E E2 1 193 69 10 = . J . That is,

    E E E hf hc hcE E2 1 2 1

    34 8

    19

    6 63 10 3 0 103 69 10

    539 = = = =

    =

    ( ) ( )

    =

    photon

    Js m / s J

    nm . .

    .

  • 24.34. Model: A particle confined in a one-dimensional box has discrete energy levels.Solve: (a) Equation 24.14 for the n = 1 state is

    E hmLn

    = =

    ( )( )( ) =

    2

    2

    34 2

    3 264

    86 63 10

    8 10 105 50 10

    .

    .

    Js kg 0.10 m

    J

    The minimum energy of the Ping-Pong ball is E1645 50 10= . J .

    (b) The speed is calculated as follows:

    E mv v v164 2 1

    23 2

    64

    3315 50 10 10 10

    2 5 50 1010 10

    3 32 10= = = ( ) = ( )

    =

    .

    .

    .J kg J

    kg m / s12

  • 24.35. Model: A particle confined in a one-dimensional box has discrete energy levels.Solve: Using Equation 24.14 for n = 1 and 2,

    E E hmL2 1

    2

    22 2

    82 1 = ( ) = ( )

    ( ) ( ) =

    1 0 108 9 11 10

    3 1 809 10192

    31 2

    37

    2..

    .

    J 6.63 10 Js

    kg Jm34 2

    L L

    =

    = =

    L 1 809 101 0 10

    1 35 1037

    199.

    .

    .

    Jm J

    m 1.35 nm2

  • 24.36. Model: A particle confined in a one-dimensional box has discrete energy levels.Solve: From Equation 24.14,

    E hmL

    nn =2

    22

    8 E h

    mLnn+ = +( )1

    2

    22

    81

    Taking the ratio,

    EE

    n

    n

    n

    n

    +=

    + 121

    Thus

    n

    n

    += = =

    1 6 43 6

    1 333 43

    .

    .

    .

    The two allowed energies have n = 3 and n = 4. Using n = 3 in the first energy equation,

    ELn

    = =( )( ) ( )

    3 6 108 9 11 10

    31934 2

    31 22

    .

    .

    J 6.63 10 Js

    kgL = 1.23 nm

  • 24.37. Model: A particle confined in a one-dimensional box of length L has the discrete energy levels given byEquation 24.14.Solve: (a) Since the energy is entirely kinetic energy,

    E hmL

    npm

    mv vhmL

    n nn n n= = = = =2

    22

    212

    2

    8 2 21 2 3, , , K

    (b) The first allowed velocity is

    v1

    34

    31 96 63 10

    2 9 11 10 0 20 10=

    ( ) ( ) =

    .

    . .

    Js kg m

    1.819 10 m / s6

    For n = 2 and n = 3, v2 = 3.64 106 m/s and v3 = 5.46 106 m/s.

  • 24.38. Model: The allowed energies of a particle of mass m in a two-dimensional square box of side L are

    E hmL

    n mnm = +( )2 2 2 28Solve: (a) The minimum energy for a particle is for n = m = 1:

    E E hmL

    hmLmin

    = = +( ) =11 2 2 2 22

    281 1

    4(b) The five lowest allowed energies are Emin , 52 Emin (for n = 1, m = 2 and n = 2, m = 1), 4Emin (for n = 2, m = 2),5Emin (for n = 1, m = 3 and n =3, m = 1), and 132 Emin (for n = 2, m = 3 and n = 3, m = 2).

  • 24.39. Model: Sets of parallel planes in a crystal diffract x-rays.Visualize: Please refer to Figure 24.7.Solve: The Bragg diffraction condition is 2d mmcos = . The plane spacing is dA = 0.20 nm and the x-raywavelength is = 0 12. nm. Thus

    cos.

    .

    . cos . . mm

    dm

    m= =( )( ) = ( ) = ( ) =

    20 12 10

    2 0 20 100 3 0 3 72 5

    9

    9 11

    AA

    m

    m

    Likewise for m = 2 and m = 3, A2 = ( ) = cos . .1 0 6 53 1 and A3 = ( ) = cos . .1 0 9 25 8 . These three angles for thex-ray diffraction peaks match the peaks shown in Figure 24.7c.(b) The new interplaner spacing is d dB A= =2 0 141. nm (see Figure 24.7b). The Bragg condition for the tiltedatomic planes becomes

    cos . mm

    dm= =

    20 4243

    B

    For m = 1, B1 = ( ) = cos . .1 0 4243 64 9 . For m = 2, B2 = ( ) = cos . .1 0 8486 31 9 .(c) The crystal is already tipped by 45 to get the tilted planes (see Figure 24.7b). So, for m = 1, 1 64 9 45= =.19 9. . = + = 64 9 45 109 9. . also, but we cant see beyond 90. For m = 2, 2 31 9 45 76 9= + = . . . These twoangles match the angles in the diffraction peaks of the tilted planes.

  • 24.40. Model: Sets of parallel planes in a crystal diffract x-rays.Visualize: Please refer to Figure CP24.40.Solve: The Bragg diffraction condition is 2d mmcos = , where d is the interplanar separation. Because smallerm values correspond to higher angles of incidence, the diffraction angle of 71.3 in the x-ray intensity plot mustcorrespond to m = 1. This means

    2 71 3 1 0 10 10 0 10 102 71 3

    1 56 1099

    10d dcos . . .cos .

    . = ( ) = ( ) =

    m m

    m

    The cosines of the three angles in the X-ray intensity plot are cos .71 3 = 0.321, cos .50 1 = 0.642, and cos .15 8 =0.962. These are in the ratio 1:2:3, which tells us that these are the m =1, 2, and 3 diffraction peaks from a singleset of planes with d = 0 156. nm.

    We can see from the figure that the atomic spacing D of this crystal is related to the interplanar separation d by

    D d=

    =

    =

    sin600.156 nm

    sin600.180 nm

  • 24.41. Model: Electrons have a de Broglie wavelength given by = h p . Trapped electrons in theconfinement layer behave like a de Broglie wave in a closed-closed tube or like a string fixed at both ends.Solve: (a) The four longest standing-wave wavelengths in the layer are = 2L , L, 23 L , and 12 L . This followsfrom the general relation for closed-closed tubes: = 2L n/ . Thus, = 10.0 nm, 5.0 nm, 3.33 nm, and 2.50 nm.(b) We have

    p mv h v hm

    = = = =

    ( ) =

    6 63 10

    9 11 100 7278 1034

    31

    3.

    .

    . Js kg

    m / s2

    Using the above four longest values of we get the four smallest values of v. Thus,

    v1

    3

    940 7278 10

    10 0 1010=

    =

    .

    .

    m / s m

    7.28 m / s2

    v2510= 1.46 m / s , v3 510= 2.18 m / s, and v4 510= 2.91 m / s .

  • 24.42. Model: As light is diffracted by matter, matter can also be diffracted by light.Visualize: Please refer to Figure CP24.42.Solve: The de Broglie wavelength of the sodium atoms is

    = = = ( )( ) =

    hp

    hmv

    6 63 103 84 10

    3 45 1034

    2610.

    .

    .

    Js kg 50 m / s

    m

    The slit spacing of the diffraction grating is d = = =12 12laser 600 nm 300 nm. Using the diffraction gratingequation with m = 1, we have

    dd

    sin sin . = ( ) = = 1 1 151 10 3 radIn the small-angle approximation, sin tan = y L . We get

    y L= = ( ) ( ) =sin . 1.0 m 1.15 mm1 151 10 3

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