Slide 1 / 99. Electromagnetic Waves

Transcription

1 Slide 1 / 99 Electromagnetic Waves

2 Slide 2 / 99 The Nature of Light: Wave or Particle The nature of light has been debated for thousands of years. In the 1600's, Newton argued that light was a stream of particles. Huygens argued it was a wave. Both had good arguments, but neither could prove their case.

3 Slide 3 / 99 1 The wave theory of light is attributed to A B C D Christian Huygens. Isaac Newton. Max Planck. Albert Einstein.

4 Slide 4 / 99 2 The particle theory of light is attributed to A B C D Christian Huygens. Isaac Newton. Max Planck. Albert Einstein.

5 Slide 5 / 99 Young's Double Slit Experiment In 1801, Thomas Young settled the argument (apparently) with his Double Slit Experiment. First, let's use what we know about sound and particles to see one way to tell the difference between particles and waves.

6 Slide 6 / 99 Young's Double Slit Experiment If two speakers are playing a sound with the same wavelength, the will constructively interfere if they travel the same distance to a screen. loud

7 Slide 7 / 99 Young's Double Slit Experiment Or, if the extra distance one sound has to travel is exactly one wavelength longer. loud

8 Slide 8 / 99 Young's Double Slit Experiment But they will destructively interfere if one sound travels half a wavelength longer than the other. quiet

9 Slide 9 / 99 Young's Double Slit Experiment So for sounds waves, we expect to get a pattern of maxima and minima like this. But this would be the case for all waves, not just sound waves. loud quiet loud quiet loud

10 Would we expect a pattern like that if two machine gunners were firing randomly at a wall, or would we expect an even distribution of bullets? Slide 10 / 99 Young's Double Slit Experiment

11 Young tested to see if light was a wave by seeing if it created an interference pattern when it went through two slits, like a wave would. Slide 11 / 99 Young's Double Slit Experiment

12 Young tested to see if light was a wave by seeing if it created an interference pattern when it went through two slits, like a wave would. Slide 12 / 99 Young's Double Slit Experiment

13 This photo is of light (of one color) striking a distant screen after passing through 2 slits. Slide 13 / 99 Young's Double Slit Experiment This only makes sense if light is a wave.

14 Slide 14 / 99 If Light is a Wave, what's waving? If light is a wave, what's waving. In sound, we know its the pressure in the air. In any simple harmonic motion, including waves, there has to be two forms, or levels, of energy and a means to move between them...what was that for light?

15 Slide 15 / 99 If Light is a Wave, what's waving? In the late 1800's James Maxwell, combined together the known equations of electricity and magnetism, and added one, to create: Maxwell's Equations Gauss's Law Gauss's Law for Magnetism Faraday's Law of Induction Ampere's Law

16 Slide 16 / 99 If Light is a Wave, what's waving? He found they predicted that energy could move between two forms (electric and magnetic) and that disturbance must travel through space at a speed of 3.0 x 10 8 m/s. This very much agreed with the known speed of light. 3.0 x 10 8 m/s is the speed of light in a vacuum.

17 Slide 17 / 99 Creating Electromagnetic Waves We already learned that a changing magnetic field produces an electric field (E = -Df B /Dt). Maxwell showed that a changing electric field produces a magnetic field as well. Once these changing fields are first started up, they keep creating each other...and travel on their own. These traveling fields are called electromagnetic waves.

18 Slide 18 / 99 Accelerating Charges create E-M waves A great way to start this up is to make a charge, like an electron accelerate. That creates a changing electric field, which creates a changing magnetic field, which creates a changing electric field, which creates a changing magnetic field which creates a changing electric field, which creates a changing magnetic field...

19 Slide 19 / 99 Accelerating Charges create E-M waves For instance, in a broadcast radio or TV antenna electrons are accelerated up and down by a changing voltage from an amplifier. As they accelerate they radiate E-M waves which travel away from the antenna.

20 Slide 20 / 99 3 An electric field is produced by a A B C D constant magnetic field. changing magnetic field. either a constant or a changing magnetic field. none of the given answers

21 Slide 21 / 99 4 A changing electric field will produce a A B C D current. gravitational field. magnetic field. none of the given answers

22 Slide 22 / 99 Electromagnetic Waves The electric and magnetic waves are perpendicular to each other, and to the direction of propagation.

23 Slide 23 / 99 Light is an Electromagnetic Wave Young showed that light is a wave. Maxwell showed that electromagnetic waves exist and travel at the speed of light. Light was shown to be an electromagnetic wave. The frequency of an electromagnetic wave is related to its wavelength. For electromagnetic waves (including light), in a vacuum: c = lf

24 Slide 24 / 99 Light is an Electromagnetic Wave

25 Slide 25 / 99 Electromagnetic Radiation All electromagnetic radiation travels at the same velocity: the speed of light (c), c = m/s. For all waves, velocity = wavelength x frequency: v = #f Therefore for light, c = lf

26 Slide 26 / 99 5 All electromagnetic waves travel through a vacuum at A B C D the same speed. speeds that are proportional to their frequency. speeds that are inversely proportional to their frequency. none of the given answers

27 6 In a vacuum, the velocity of all electromagnetic waves Slide 27 / 99 A B C D is zero. is m/s. depends on the frequency. depends on their amplitude.

28 Slide 28 / 99 7 Of the following, which is not electromagnetic in nature? A B C D microwaves gamma rays sound waves radio waves

29 8 Which of the following correctly lists electromagnetic waves in order from longest to shortest wavelength? Slide 29 / 99 A B C D gamma rays, ultraviolet, infrared, microwaves microwaves, ultraviolet, visible light, gamma rays radio waves, infrared, gamma rays, ultraviolet television, infrared, visible light, X-rays

30 Slide 30 / 99 9 For a wave, the frequency times the wavelength is the wave's A B C D speed. amplitude. intensity. power.

31 Slide 31 / What color of light has the shortest wavelength? A B C D Green Red Yellow Blue

32 11 What color of light has the longest wavelength? Slide 32 / 99 A B C D Green Red Yellow Blue

33 12 Electromagnetic radiation travels through vacuum at a speed of Slide 33 / 99 A B C D 186,000 m/s 125 m/s 3.00 x 10 8 m/s It depends on wavelength

34 Slide 34 / The wavelength of light that has a frequency of 1.20 x s is A B C D 25 m 2.5 x 10-5 m m 2.5 m c = lf c = m/s

35 14 What is the frequency of light whose wavelength is 600 nm? Slide 35 / 99 A B C D 5.0 x Hz 1.0 x Hz 1.5 x Hz 2.0 x Hz c = lf c = m/s

36 Slide 36 / 99 The Visible Spectrum Wavelengths of visible light: 400 nm to 750 nm Shorter wavelengths are ultraviolet; longer wavelengths are infrared UV IR λ f 400 nm 500 nm 600 nm 700 nm 7.5 x Hz 6 x Hz 5 x Hz 4 x Hz

37 Slide 37 / Visible light ranges in wavelength from A B C D 400 μm to 750 μm. 400 nm to 750 nm. 500 μm to 850 μm. 500 nm to 850 nm.

38 Slide 38 / White light is A B C D E light of wavelength 550 nm, in the middle of the visible spectrum. a mixture of all frequencies. a mixture of red, green, and blue light. the term used to describe very bright light. the opposite (or complementary color) of black light.

39 Slide 39 / Light with wavelength slightly longer than 750 nm is called A B C D ultraviolet light. visible light. infrared light. none of the given answers

40 Slide 40 / 99 Interference Young s Double-Slit Experiment The double slit experiment relies on two properties of waves (including light): diffraction and interference. Each slit generates a new wave due to diffraction. Those waves then either constructively or destructively interfere on a faraway screen. by Patrick Edwin Moran by Francesco Franco

41 Slide 41 / 99 Waves Versus Particles: Huygens Principle Every point on a wave front acts as a point source; the wavefront as it develops is tangent to their envelope

42 Slide 42 / 99 Diffraction When waves encounter an obstacle, they bend around it, leaving a shadow region. This is called diffraction. Exploratorium, Some rights reserved. Unless otherwise noted, this work is licensed under creativecommons.org/licenses/by-nc-sa/3.0/us/

43 Slide 43 / 99 Diffraction When waves, including light, meets an obstacle it bends around it to some extent. When it meets a small opening, the opening generates a new wave on the other side.

44 Slide 44 / What principle is responsible for light spreading as it passes through a narrow slit? A B C D refraction polarization diffraction interference

45 Slide 45 / What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? A B C D refraction polarization dispersion interference

46 Slide 46 / If a wave from one slit of a Young's double slit experiment arrives at a point on the screen onehalf wavelength behind the wave from the other slit, which is observed at that point? A B C D bright fringe dark fringe gray fringe multi-colored fringe

47 Slide 47 / 99 Double-Slit Maxima and Minima Interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot). by Francesco Franco

48 Slide 48 / 99 Double-Slit Maxima and Minima d L Extra distance = # x The bright lines that appear on the screen are called maxima. The dark lines are called minima. Maxima are evenly spaced, and a minimum occurs between each pair of maxima. The distance to the first maxima can be found by using similar triangles.

49 Slide 49 / 99 Interference of Light Waves L d bright spot dark spot bright spot θ 1 θ 2 dark spot bright spot dark spot bright spot dark spot bright spot A constructive interference pattern is given by: d sin# = m# A destructive interference pattern is given by: d sin# = (m + ½)# Where m is called the order of the interference fringe.

50 Slide 50 / 99 Interference of Light Waves L bright spot d θ 1 θ 2 x bright spot bright spot bright spot bright spot For small angles, θ<10, tan θ = sin θ. Since tanθ = x/l, sinθ = x/l... d sinθ = mλ becomes: dx/l = mλ

51 Slide 51 / 99 Double-Slit Maxima and Minima x # mll d x # (m + 1/2)l L d The maxima and minima spread out as the distance between the slits gets smaller. As d gets smaller...x gets larger.

52 Slide 52 / 99 Double-Slit Maxima and Minima Brightness versus distance (x) from the central maximum is plotted below. Between the maxima and the minima, the interference varies smoothly. Constructive interference Destructive interference

53 Slide 53 / 99 Interference - Young's Double Slit Experiment Since the position of the maxima (except for the central one) depends on wavelength, the first and high-order fringes contain a spectrum of colors.

54 Slide 54 / 99 Diffraction Grating A diffraction grating consists of a large number of equally spaced narrow slits or lines. They produce maxima and minima, just like in the Double Slit experiment, but the pattern is much sharper because there are thousands of slits, not just two. The more lines or slits there are, the narrower the peaks. Also, shining white light on the grating produces spectra of colors since the location of maxima depends on wavelength.

55 Slide 55 / 99 Diffraction Grating The maxima of the diffraction pattern on a far away screen is the same as it was for two slits, the lines are just brighter and sharper. x # mll d

56 Slide 56 / What happens to a diffraction pattern if the wavelength of the light is decreased? A B C D Interference fringes move closer to the central maximum. Interference fringes move away from the central maximum. There is no change in the interference. Bright fringes are replanced with dark fringes.

57 Slide 57 / What happens to a diffraction pattern if the space between the slits is decreased? A B C D Interference fringes move closer to the central maximum. Interference fringes move away from the central maximum. There is no change in the interference. Bright fringes are replanced with dark fringes.

58 Slide 58 / 99 Single Slit Interference When light strikes even a single slit, interference occurs between light at the center of the slit with light at the bottom...and top. D D

59 Slide 59 / 99 Single Slit Interference In this case, d (from the equation for single slit interference) becomes 1/2D (the distance from the top of the slit to its center. So the equation for the first minimum (m=0) becomes: x # (m + 1/2)l L d m = 0, 1, 2,... D x # x # 1/2l L 1/2D l L D

60 Slide 60 / 99 Single Slit Interference The resulting pattern of light and dark stripes is called a diffraction pattern. The width of the central maximum is 2l/D. As D gets smaller, the central maximum becomes wider. As D gets larger, the central maximum gets smaller. -3lL D -2lL D -ll D 0 ll D 2lL D 3lL D

61 Slide 61 / 99 Single Slit Interference The width of the central maximum is important for optical instruments (including our eyes) as it limits how clearly we see. The wider the central maximum is, the more smeared out objects appear...the less we can resolve one object from another. That's why an eagle's eye is so large. Why large lenses on cameras give better pictures...why telescopes have to be large, etc. As D gets very large the more clear the image we see.

62 Slide 62 / 99 Diffraction Interference Around an Object Light also bends around objects, creating a bright spot where it would be least expected.

63 Slide 63 / What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? A B C D refraction polarization dispersion interference

64 Slide 64 / If a wave from one slit of a Young's double slit experiment arrives at a point on the screen onehalf wavelength behind the wave from the other slit, which is observed at that point? A B C D bright fringe dark fringe gray fringe multi-colored fringe

65 Slide 65 / The separation between adjacent maxima in a double-slit interference pattern using monochromatic light is A B C D greatest for red light. greatest for green light. greatest for blue light. the same for all colors of light.

66 Light slows when traveling through a medium. The index of refraction (n) of the medium is the ratio of the speed of light in vacuum to the speed of light in the medium: Slide 66 / 99

67 Slide 67 / Light travels fastest A B C D in a vacuum. through water. through glass. through diamond.

68 Slide 68 / For all transparent material substances, the index of refraction A is less than 1. B is greater than 1. C is equal to 1. D could be any of the given answers; it all depends on optical density.

69 Slide 69 / The index of refraction of diamond is This means that a given type of light travels A B C D 2.42 times faster in air than it does in diamond times faster in diamond than it does in air times faster in vacuum than it does in diamond times faster in diamond than it does in vacuum.

70 Slide 70 / 99 The frequency of the light does not change, but the wavelength does as it travels into a new medium. where "n" is the index of refraction. Wavelengths get shorter when light enters a slower medium.

71 Slide 71 / When a light wave enters into a medium of different optical density, A B C D its speed and frequency change. its speed and wavelength change. its frequency and wavelength change. its speed, frequency, and wavelength change.

72 Slide 72 / When a beam of light (wavelength = 590 nm), originally traveling in air, enters a piece of glass (index of refraction 1.50), its frequency A increases by a factor of B is reduced to 2/3 its original value. C is unaffected. D none of the given answers

73 Slide 73 / When a beam of light (wavelength = 590 nm), originally traveling in air, enters a piece of glass (index of refraction 1.50), its wavelength A increases by a factor of B is reduced to 2/3 its original value. C is unaffected. D none of the given answers

74 Slide 74 / When a light wave enters into a medium of different optical density, A B C D its speed and frequency change. its speed and wavelength change. its frequency and wavelength change. its speed, frequency, and wavelength change.

75 Slide 75 / When a beam of light (wavelength = 590 nm), originally traveling in air, enters a piece of glass (index of refraction 1.50), its frequency A increases by a factor of B is reduced to 2/3 its original value. C is unaffected. D none of the given answers

76 Slide 76 / When a beam of light (wavelength = 590 nm), originally traveling in air, enters a piece of glass (index of refraction 1.50), its wavelength A increases by a factor of B is reduced to 2/3 its original value. C is unaffected. D none of the given answers

77 Slide 77 / 99 Dispersion The index of refraction of a material varies somewhat with the wavelength of the light.

78 Slide 78 / 99 Dispersion This variation in refractive index is why a prism will split white light (which contains all the colors) into a rainbow of colors.

79 Slide 79 / White light is A B C D E light of wavelength 550 nm, in the middle of the visible spectrum. a mixture of all frequencies. a mixture of red, green, and blue light. the term used to describe very bright light. the opposite (or complementary color) of black light.

80 Slide 80 / The principle which explains why a prism separates white light into different colors is A B C D refraction. polarization. dispersion. total internal reflection.

81 Slide 81 / Which color of light undergoes the smallest refraction when passing from air to glass? A B C D red yellow green violet

82 Slide 82 / 99 The Visible Spectrum and Dispersion Actual rainbows are created by dispersion in tiny drops of water. Copyright RichTea and licensed for reuse under this Creative Commons Licence. Copyright Beyonder and licensed for reuse under this Creative Commons Licence.

83 Slide 83 / The principle which allows a rainbow to form is A B C D refraction. polarization. dispersion. total internal reflection.

84 Slide 84 / Light with wavelength slightly shorter than 400 nm is called A B C D ultraviolet light. visible light. infrared light. none of the given answers

85 Slide 85 / Which color of light undergoes the greatest refraction when passing from air to glass? A B C D red yellow green violet

86 Slide 86 / 99 Interference by Thin Films The colors on the soap bubble are created by interference by thin films.

87 Slide 87 / 99 Interference by Thin Films Consider a smooth surface of water with a thin film of oil on top of it. The oil's index of refraction is less than that of water. Part of the incident light is reflected at point A, and part of it is reflected at point B. The part reflected at the lower surface must travel the extra distance ABC in the oil. If t is the thickness of the film then ABC is equal to 2t. A B C Air Oil Water

88 Slide 88 / 99 Interference by Thin Films If that distance is equal to λ, 2λ, 3λ, and so on then the waves will interfere constructively. 2t = mλ, where m = 1, 2, 3... If that distance is equal to λ/2, 3λ/2, 5λ/2, and so on then the waves will interfere destructively. A B C Air Oil Water 2t = (m+½) λ, where m = 1, 2, 3... n air < n oil < n water The wavelength, λ, is the wavelength in the film of oil and t is the thickness of the film.

89 Slide 89 / 99 Interference by Thin Films If that distance is equal to λ, 2λ, 3λ, and so on then the waves will interfere constructively. 2t = (m+½)λ, where m = 1, 2, 3... If that distance is equal to λ/2, 3λ/2, 5λ/2, and so on then the waves will interfere destructively. A B C Air Film Air 2t = mλ, where m = 1, 2, 3... n air < n water > n air The wavelength, λ, is the wavelength in the film of oil and t is the thickness of the film.

90 Slide 90 / The colors on an oil slick are caused by reflection and A B C D diffraction. interference. refraction. polarization.

91 Slide 91 / A light with a wavelength of 500nm shines on a glass block that is covered by a thin film n = 1.2. What must be the minimum thickness of the film in order to minimize the intensity of the reflected light?

92 Slide 92 / A light with a wavelength of 500nm shines on a glass block that is covered by a thin film n = 1.2. What must be the minimum thickness of the film in order to maximize the intensity of the reflected light?

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96 Slide 96 / Electromagnetic waves are A B C D longitudinal. transverse. both longitudinal and transverse. neither longitudinal or transverse.

97 Slide 97 / 99 Polarization Because the intensity of a light beam is proportional to the square of the amplitude, the intensity of a plane-polarized beam transmitted by a polarizer is: I = I 0 cos 2 θ where θ is the angle between the polarizer axis and the plane of polarization and I 0 is the incoming intensity. Note that the incoming light in this equation is already polarized. When light travels through only one polarizer then intensity is reduced to one-half the original.

98 Slide 98 / What principle is responsible for the fact that certain sunglasses can reduce glare from reflected surfaces? A B C D refraction polarization diffraction total internal reflection

99 Slide 99 / Unpolarized light passes through two polarizers the axis of one is vertical and the axis of the other is tilted 30 degrees from the vertical. If the incomming intensity is I0, what is the intensity of the transmitted light? A I 0 /4 B I 0 /4 C 3I 0 /8 D 3I 0 /4