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Unformatted text preview: 1010 CHAPTER 37 DIFFRACTION CHAPTER 37 Answer to Checkpoint Questions 1. 2. 3. 4. 5. 6. (a) expand; (b) expand (a) second side maximum; (b) 2:5 (a) red; (b) violet diminish (a) increase; (b) same (a) left; (b) less Answer to Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. (a) contract; (b) contract (a) the m = 5 minimum; (b) (approximately) the maximum between the m = 4 and m = 5 minima with megaphone (larger opening, less diraction) (a) a and c tie, then b and d tie; (b) a and b tie , then c and d tie four red (a) larger; (b) red (a) less; (b) greater; (c) greater (a) decrease; (b) same; (c) in place (a) decrease; (b) decrease; (c) to the right (a) A; (b) left; (c) left; (d) right CHAPTER 37 DIFFRACTION 1011 12. (a) increase; (b) rst order Solutions to Exercises & Problems 1E a sin = m, where a is the slit width, is the wavelength, and m is an integer. Solve for : = a sin =m = (0:022 mm)(sin 1:8 )=1 = 6:9 10 3 mm = 690 nm: (a) = sin 1 (1:50 cm=2:00 m) = 0:430 : (b) For the mth diraction minimum a sin = m: Solve for a: nm) m a = sin = 2(441430 = 0:118 mm : sin 0: Use a sin = m. The angle is measured from the forward direction, so for the situation described in the problem it is 0:60 , for m = 1. Thus 9 m a = sin = 633 010 m = 6:04 10 5 m : sin :60 (a) Use a sin = m. For = a and m = 1 the angle is the same as for = b and m = 2. Thus a = 2b . (b) Let ma be the integer associated with a minimum in the pattern produced by light with wavelength a and let mb be the integer associated with a minimum in the pattern produced by light with wavelength b . A minimum in one pattern coincides with a minimum in the other if they occur at the same angle. This means ma a = mb b . Since a = 2b , the minima coincide if 2ma = mb . Thus every other minimum of the b pattern coincides with a minimum of the a pattern. (a) Use Eq. 37-3 to calculate the separation between the rst (m1 = 1) and fth (m2 = 5) minima: m = D m = D (m m ) : y = D sin = D a a a 2 1 5E 4E 3E 2E The condition for a minimum in a single-slit diraction pattern is given by Eq. 37-3: 1012 CHAPTER 37 DIFFRACTION solve for a: 6 a = D(m2 y m1 ) = (400 mm)(550: 10 mm)(5 1) = 2:5 mm : 0 35 mm (b) For m = 1 The angle is = sin 1 (2:2 10 4 ) = 2:2 10 4 rad: 6E 10 6 sin = m = (1)(550 : mm mm) = 2:2 10 4 : a 25 From Eq. 37-3 a = m = 1 = 1:41 : sin sin 45:0 (a) A plane wave is incident on the lens so it is brought to focus in the focal plane of the lens, a distance of 70 cm from the lens. (b) Waves leaving the lens at an angle to the forward direction interfere to produce an intensity minimum if a sin = m, where a is the slit width, is the wavelength, and m is an integer. The distance on the screen from the center of the pattern to the minimum is given by y = D tan , where D is the distance from the lens to the screen. For the conditions of this problem 9 m) sin = m = (1)(590 10 3 m = 1:475 10 3 : a 0:40 10 7E This means = 1:47510 3 rad and y = (7010 2 m) tan(1:47510 3 rad) = 1:010 3 m. 8P The condition for a minimum of intensity in a single-slit diraction pattern is a sin = m, where a is the slit width, is the wavelength, and m is an integer. To nd the angular position of the rst minimum to one side of the central maximum set m = 1: 1 = sin 1 = sin a 1 589 10 9 m = 5:89 10 4 rad : 1:00 10 3 m If D is the distance from the slit to the screen, the distance on the screen from the center of the pattern to the minimum is y1 = D tan 1 = (3:00 m) tan(5:89 10 4 rad) = 1:767 10 3 m. CHAPTER 37 DIFFRACTION 1013 To nd the second minimum set m = 2: 2 = sin 1 2(589 10 9 m) = 1:178 10 3 rad : 1:00 10 3 m The distance from the pattern center to the minimum is y2 = D tan 2 = (3:00 m) tan(1:178 10 3 rad) = 3:534 10 3 m. The separation of the two minima is y = y2 y1 = 3:534 mm 1:767 mm = 1:77 mm. 9P Let the rst mimimum be a distance y from the central axis which is perpendicular to the speaker. Then sin = y=(D2 + y2 )1=2 = m=a = =a (for m = 1). Slove for y: y = p D2 = p D 2 (a=) 1 (af=vs ) 1 100 m = 41:2 m : =p [(0:300 m)(3000 Hz)=(343 m/s)]2 1 10P From y = mD=a we get : nm)(2 y = mD = D m = (6321837 mm :60) [10 ( 10)] = 24:0 mm : a a : 11E From Eq. 37-4 2 = 2 (x sin ) = 589 nm 0:10 mm (sin 30 ) = 267 rad : 2 This is equivalent to 267 rad 84 = 2:79 rad = 160 : 12E (a) = sin 1 (1:1 cm=3:5 m) = 0:18 : (b) Use Eq. 37-6: mm)(sin = a sin = (0:025 538 nm 0:18 ) = 0:46 rad : 1014 CHAPTER 37 DIFFRACTION If you divide the original slit into N strips and represent the light from each strip, when it reaches the screen, by a phasor, then at the central maximum in the diraction pattern you add N phasors, all in the same direction and each with the same amplitude. The intensity there is proportional to N 2 . If you double the slit width you need 2N phasors if they are each to have the amplitude of the phasors you used for the narrow slit. The intensity at the central maximum is proportional to (2N )2 and is therefore 4 times the intensity for the narrow slit. The energy reaching the screen per unit time, however, is only twice the energy reaching it per unit time when the narrow slit is in place. The energy is simply redistributed. For example, the central peak is now half as wide and the integral of the intensity over the peak is only twice the analogous integral for the narrow slit. 14P 13P Think of the Huygens' explanation of diraction phenomenon. When A is in place only the Huygens' wavelets that pass through the hole get to point P . Suppose they produce a resultant electric eld EA . When B is in place the light that was blocked by A gets to P and the light that passed through the hole in A is blocked. Suppose the electric eld at P is now EB . The sum EA + EB is the resultant of all waves that get to P when neither A nor B are present. Since P is in the geometric shadow this is zero. Thus EA = EB and since the intensity is proportional to the square of the electric eld, the intensity at P is the same when A is present as when B is present. (a) The intensity for a single-slit diraction pattern is given by sin2 ; I = Im 2 15P where = (a=) sin , a is the slit width and is the wavelength. The angle is measured from the forward direction. You want I = Im =2, so 1 sin2 = 2 2 : (b) Evaluate sin2 and 2 =2 for = 1:39 rad and compare the results. To be sure that 1:39 rad is closer to the correct value for than any other value with 3 signi cant digits, you should also try 1:385 rad and 1:395 rad. (c) Since = (a=) sin , 1 : = sin a Now = = 1:39= = 0:442, so = sin 1 0:442 : a CHAPTER 37 DIFFRACTION 1015 The angular separation of the two points of half intensity, one on either side of the center of the diraction pattern, is = 2 = 2 sin (d) For a= = 1:0, for a= = 5:0, and for a= = 10, 1 0:442 : a = 2 sin 1 (0:442=1:0) = 0:916 rad = 53 ; = 2 sin 1 (0:442=5:0) = 0:177 rad = 10 ; = 2 sin 1 (0:442=10) = 0:0884 rad = 5:1 : 16P (a) The intensity for a single-slit diraction pattern is given by I = Im sin 2 ; where = (a=) sin . Here a is the slit width and is the wavelength. To nd the maxima and minima, set the derivative of I with respect to equal to zero and solve for . The derivative is dI = 2I sin ( cos sin ) : 2 These are the maxima. (b) The values of that satisfy tan = can be found by trial and error on a pocket calculator or computer. Each of them is slightly less than one of the values (m + 1 ) rad, so start with these values. The rst 2 few are 0, 4:4934, 7:7252, 10:9041, 14:0662, and 17:2207. They can also be found graphically. As in the diagram to the right, plot y = tan and y = on the same graph. The intersections of the line with the tan curves are the solutions. The rst two solutions listed above are shown on the diagram. 3 The derivative vanishes if 6= 0 but sin = 0. This yields = m, where m is an integer. Except for m = 0 these are the intensity minima: I = 0 for = m. The derivative also vanishes for cos sin = 0. This condition can be written tan = . y y=tan d m y= 0 /2 3 /2 (rad) 1016 CHAPTER 37 DIFFRACTION (c) Write = (m + 1 ) for the maxima. For the central maximum, = 0 and m = 2 For the next, = 4:4934 and m = 0:930. For the next = 7:7252 and m = 1:959. 17P 1. 2 Since the slit width is much less than the wavelength of the light, the central peak of the single-slit diraction pattern is spread across the screen and the diraction envelope can be ignored. Consider 3 waves, one from each slit. Since the slits are evenly spaced the phase dierence for waves from the rst and second slits is the same as the phase dierence for waves from the second and third slits. The electric elds of the waves at the screen can be written E1 = E0 sin(!t), E2 = E0 sin(!t + ), and E3 = E0 sin(!t + 2), where = (2d=) sin . Here d is the separation of adjacent slits and is the wavelength. The phasor diagram is shown to the right. It yields E = E0 cos + E0 + E0 cos = E0 (1 + 2 cos ) for the amplitude of the resultant wave. Since the intensity of a wave is proportional to the square of the 2 electric eld, we may write I = AE0 (1+2 cos )2 , where A is a constant of proportionality. If Im is the intensity at the center of the pattern, for which = 0, then 2 2 Im = 9AE0 . Take A to be Im =9E0 and obtain m m I = I9 (1 + 2 cos )2 = I9 1 + 4 cos + 4 cos2 : E3 E2 E t E1 Use Eq. 37-12: sin = 1:22=d. In our case = 2:5 =2 = 1:25 , so 18E :22 d = 1sin = 1:22(550 nm) = 31 m : sin 1:25 (a) Use the Rayleigh criteria. To resolve two point sources the central maximum of the diraction pattern of one must lie at or beyond the rst minimum of the diraction pattern of the other. This means the angular separation of the sources must be at least R = 1:22=d, where is the wavelength and d is the diameter of the aperture. For the headlights of this problem 9 R = 1:22(550 10 m m) = 1:34 10 4 rad : 5:0 10 3 19E (b) If L is the distance from the headlights to the eye when the headlights are just resolvable and D is the separation of the headlights, then D = L tan R LR , where the small angle CHAPTER 37 DIFFRACTION 1017 approximation tan R R was made. This is valid if R is measured in radians. Thus L = D=R = (1:4 m)=(1:34 10 4 rad) = 1:0 104 m = 10 km. 20E (a) Use Eq. 37-14: 6 R = 1:22 = (1:22)(5400 10 mm) = 1:3 10 4 rad : d 5: mm (b) The linear separation is D = LR = (160 103 m)(1:3 10 4 rad) = 21 m: 21E The minimum separation is 8 9 Dmin = dem R = dem 1:22 = (1:22)(3:82 10 :1m)(550 10 m) = 50 m : d 5 m 22E 3 3 D D :0 Lmax = = 1:22=d = (5:0 :10 m)(410 10 m) = 30 m : 1 22(550 9 m) R 23E 1:22 = (1:22)(250 mm)(500 10 3 m) = 30:5 m : Dmin = LR = L d 5:00 mm 24E (a) Use Rayleigh's criterion: two objects can be resolved if their angular separation at the observer is greater than R = 1:22=d, where is the wavelength of the light and d is the diameter of the aperture (the eye or mirror). If L is the distance from the observer to the objects then the smallest separation D they can have and still be resolvable is D = L tan R LR , where R is measured in radians. The small angle approximation tan R R was made. Thus 10 9 D = 1:22L = 1:22(8:0 510 m)(550 10 m) = 1:1 107 m = 1:1 104 km : d :0 10 3 m This distance is greater than the diameter of Mars. One part of the planet's surface cannot be resolved from another part. 1018 CHAPTER 37 DIFFRACTION (b) Now d = 5:1 m and 10 9 D = 1:22(8:0 10 5:m)(550 10 m) = 1:1 104 m = 11 km : 1m 25E D = D = (5:0 10 2 m)(4:0 10 3 m) = 1:6 106 m = 1600 km : Lmax = 1:22(0:10 10 9 m) R 1:22=d 26E 3 2 Dmin = LR = L 1:22 = (6:2 10 m)(1::22)(1:6 10 m) = 53 m : d 2 3m 27P (a) The diameter is 3 9 :22)(1 D = LR = L 1:22 = (2000 100:m)(1 10 3:40 10 m) = 17:1 m : d 200 m (b) I=Im = (d=D)2 = (0:200 10 3 m=1:71 m)2 = 1:37 10 28P 10 : D = 2(50 10 6 m)(1:5 10 3 m) = 0:19 m : Lmax = 1:22=d 1:22(650 10 9 m) (b) The wavelength of the blue light is shorter so Lmax / 1 will be larger. According to Rayleigh's criterion (Eq. 37-14) R = 1:22=d. In our case R D=L, where D = 60 m is the size of the object your eyes must resolve, and L is the limiting viewing distance in question. Also d = 3:00 mm is the diameter of your pupil. Solve for L: 29P (a) L = 1Dd = (60 m)(3:00 mm) = 27 cm : :22 1:22(550 nm) CHAPTER 37 DIFFRACTION 1019 30P (a) Use Eq. 37-12: = sin = sin (b) Now f = 1:0 103 Hz so 1:22 = 1 1 (1:22)(1450 m/s) = 6:8 : (25 103 Hz)(0:60 m) 1:22 = sin d 1 1:22(vs =f ) d d (1:22)(1450 m/s) = 2:9 > 1 : (1:0 103 Hz)(0:60 m) Since sin cannot exceed 1 there is no minimum. 31P Use 2 = 1:22=d = D=L: 9 d = 1:22L = (1:22)(220 mi)(1610 :m/mi)(500 10 m) = 4:7 cm : D (30 ft)(0 305 m/ft) 32P From R = 1:22=d = D=L we get 9 3 d = 1:22L = (1:22)(550 10:30m)(160 10 m) = 0:36 m : D 0 m (a) The rst minimum in the diraction pattern is at an angular position , measured from the center of the pattern, such that sin = 1:22=d, where is the wavelength and d is the diameter of the antenna. If f is the frequency then the wavelength is = c=f = (3:00 108 m/s)=(220 109 Hz) = 1:36 10 3 m. Thus 33P = sin 1 1:22 = sin d 1 1:22(1:36 10 3 m) = 3:02 10 3 rad : 55:0 10 2 m The angular width of the central maximum is twice this, or 6:04 10 3 rad (0:346 ). 1020 CHAPTER 37 DIFFRACTION (b) Now = 1:6 cm and d = 2:3 m, so 2 1 1:22(1:6 10 m) = 8:5 10 3 rad : = sin 2:3 m The angular width of the central maximum is 1:7 10 2 rad (0:97 ). (a) The angular separation is 9 = R = 1:22 = (1:22)(550 10 m) = 8:8 10 7 rad = 0:1800 : d 0:76 m (b) The distance is 1012 km/ly)(0:18) = 8:4 107 km : D = LR = (10 ly)(9:46(3600)(180) (c) The diameter is d = 2R L = 2(0:18)()(14 m) = 2:5 10 5 m = 0:025 mm : (3600)(180) 35P 34P (a) Since = 1:22=d, the larger the wavelength the larger the radius of the rst minimum (and second maximum, ect). Therefore the white pattern is outlined by red lights (with longer wavelength than blue lights). (b) 1:22(7 10 7 d = 1:22 1:5(0:50 =180m)=2 = 1:3 10 4 m : ) )( The energy of the beam of light which is projected onto the moon is concentrated is a circular spot of diameter d1 , where d1 =dem = 2 = 2(1:22=d0 ), with d0 the diameter of the mirror on Earth and dem the Earth-Moon separation. The fraction of energy picked up by the re ector of diameter d2 on the Moon is then 0 = (d2 =d1 )2 . This re ected light, upon reaching the Earth, has a circular cross section of diameter d3 satisfying d3 =dem = 2 = 2(1:22=d2 ). The fraction of the re ected energy that is picked up by the telescope is then 00 = (d0 =d3 )2 : Thus the fraction of the original energy picked up by the detector is 2 2 d2 = 0 00 = d0 = 36P 4 (2:6 m)(0:10 m) = 2:44(0:69 10 6 m)(3:82 108 m) 4 10 d3 d1 2 d0 d2 d d2 = 2:440d (2:44dem =d0 )(2:44dem =d2 ) em 4 13 : CHAPTER 37 DIFFRACTION 1021 Bright interference fringes occur at angles given by d sin = m, where d is the slit separation, is the wavelength, and m is an integer. For the slits of this problem d = 11a=2, so a sin = 2m=11. The rst minimum of the diraction pattern occurs at the angle 1 given by a sin 1 = and the second occurs at the angle 2 given by a sin 2 = 2, where a is the slit width. You want to count the values of m for which 1 < < 2 , or what is the same, the values of m for which sin 1 < sin < sin 2 . This means 1 < (2m=11) < 2. The values are m = 6, 7, 8, 9, and 10. There are ve bright fringes in all. 38E 37E The number is 2(d=a) 1 = 2(2a=a) 1 = 3: It is clear from Eq. 37-5 that for a single slit of width 2a the diraction pattern is given by 39E I = Im sin(2) ; 2 where = a sin =. Now, if we put d = a in Eq. 37-19, then = = a sin =, and Eq. 37-19 reduces to I = Im (cos )2 sin 2 = Im 2 sin cos 2 2 = Im sin(2) 2 2 ; where the trigonometric identity sin(2) = 2 sin cos was used. Thus Eq. 37-19 indeed reduces to the diraction pattern for a single slit of width 2a. (a) Let the location of the fourth bright fringe coincide with the rst minimum of diraction pattern: sin = 4=d = =a, or d = 4a. (b) Any bright fringe which happens to be at the same location with a diraction minimum will vanish. So let sin = m1 =d = m2 =a = m1 =4a = m2 =a, or m1 = 4m2 where m2 = 1; 2; 3; : The fringes missing are thus the 4th, the 8th, the 12th, , i.e., every fourth fringe is missing. The angular location of the mth bright fringe is given by d sin = m so the linear separation between two adjacent fringe is m y = (D sin ) = Dd = D m = D : d d 41P 40P 1022 CHAPTER 37 DIFFRACTION (a) The angular positions of the bright interference fringes are given by d sin = m, where d is the slit separation, is the wavelength, and m is an integer. The rst diraction minimum occurs at the angle 1 given by a sin 1 = , where a is the slit width. The diraction peak extends from 1 to +1 , so you want to count the number of values of m for which 1 < < +1 , or what is the same, the number of values of m for which sin 1 < sin < + sin 1 . This means 1=a < m=d < 1=a or d=a < m < +d=a. Now d=a = (0:150 10 3 m)=(30:0 10 6 m) = 5:00, so the values of m are m = 4, 3, 2, 1, 0, +1, +2, +3, and +4. There are nine fringes. (b) The intensity at the screen is given by 42P I = Im cos2 sin 2 ; where = (a=) sin , = (d=) sin , and Im is the intensity at the center of the pattern. For the third bright interference fringe d sin = 3, so = 3 rad and cos2 = 1. Similarly, = 3a=d = 3=5:00 = 0:600 rad and (sin )2 =2 = (sin 0:600)2 =(0:600)2 = 0:255. The intensity ratio is I=Im = 0:255. (a) The rst minimum of the diraction pattern is at 5:00 so a = = sin = 0:440 m= sin 5:00 = 5:05 m. (b) Since the fourth bright fringe is missing d = 4a = 4(5:05 m) = 20:2 m: (c) For the m = 1 bright fringe sin = a sin = (5:05 :m)m 1:25 = 0:787 rad ; 0 440 so the intensity of the m = 1 fringe is 43P I = Im sin 2 = (7:0 mW/cm2 ) sin 0:787 rad 0:787 2 = 5:7 mW/cm2 ; which agrees with what Fig. 37-39 indicates. Similarly for m = 2 I = 2:9 mW/cm2 , also in agreement with Fig. 37-39. 44P As the phase dierence is varied from zero to , both the intensity pro le of the diraction and the location of the interference maximum change. At = , the original central diraction envelop is now a minimum, and the two maxima of the diraction intensity pro le are now centered where the rst minima were. The locations of the intensity maxima/minima due to interference exchange, with the original locations of the maxima now those of minima, and vice versa. CHAPTER 37 DIFFRACTION 1023 As is further varied from to 2, the intensity pattern is gradually changed back, resuming the original pattern at = 2. (a) d = 20:0 mm=6000 = 0:00333 mm = 3:33 m: (b) Let d sin = m (m = 0; 1; 2; ), we nd = 0 for m = 0, = sin 1 (=d) = sin 1 (0:589 m= 3:30 m) = 10:2 for m = 1, and similarly 20:7 for m = 2; 32:2 for m = 3; 45 for m = 4, and 62:2 for m = 5. Since jmj=d > 1 for jmj 6 these are all the maxima. 46E 45E (a) Let d sin = m and solve for : = d sin = (1:0 mm=200)(sin 30 ) = 2500 nm ; m m m where m = 1; 2; 3 . In the visible light range m can assume the following values: m1 = 4; m2 = 5 and m3 = 6. The corresponding wavelengths are 1 = 2500 nm=4 = 625 nm; 2 = 2500 nm=5 = 500 nm, and 3 = 2500 nm=6 = 416 nm. (b) The colors are orange (for 1 = 625 nm), blue-green (for 2 = 500 nm), and violet (for 3 = 416 nm). The angular location of the mth order diraction maximum is given by m = d sin . To be able to observe the fth-order one we must let sin jm=5 = 5=d < 1, or 47E < d = 1:00 nm=315 = 635 nm : 5 5 So all wavelengths shorter than 635 nm can be used. The ruling separation is d = 1=(400 mm 1 ) = 2:5 10 3 mm. Diraction lines occur at angles such that d sin = m, where is the wavelength and m is an integer. Notice that for a given order the line associated with a long wavelength is produced at a greater angle than the line associated with a shorter wavelength. Take to be the longest wavelength in the visible spectrum (700 nm) and nd the greatest integer value of m such that is less than 90 . That is, nd the greatest integer value of m for which m < d. Since d= = (2:5 10 6 m)=(700 10 9 m) = 3:57 that value is m = 3. There are 3 complete orders on each side of the m = 0 order. The second and third orders overlap (see 59P). 48E 1024 CHAPTER 37 DIFFRACTION 49E Let the total number of ...
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