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Slide 1 / 99 Electromagnetic Waves The Nature of Light: Wave or Particle Slide 2 / 99 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. 1 The wave theory of light is attributed to A Christian Huygens. B C Isaac Newton. D Albert Einstein. Max Planck. Slide 3 / 99 2 Slide 4 / 99 The particle theory of light is attributed to A Christian Huygens. B C Isaac Newton. D Albert Einstein. Max Planck. 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. 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 Slide 7 / 99 Young's Double Slit Experiment Or, if the extra distance one sound has to travel is exactly one wavelength longer. loud 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 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 Slide 9 / 99 Young's Double Slit Experiment Slide 10 / 99 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? Young's Double Slit Experiment Slide 11 / 99 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. Young's Double Slit Experiment 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 Slide 13 / 99 This photo is of light (of one color) striking a distant screen after passing through 2 slits. This only makes sense if light is a wave. If Light is a Wave, what's waving? Slide 14 / 99 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? 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 Slide 15 / 99 If Light is a Wave, what's waving? Slide 16 / 99 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 108 m/s. This very much agreed with the known speed of light. 3.0 x 108 m/s is the speed of light in a vacuum. Creating Electromagnetic Waves Slide 17 / 99 We already learned that a changing magnetic field produces an electric field (E = -DfB/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. 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...... Slide 18 / 99 Accelerating Charges create E-M waves Slide 19 / 99 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. 3 An electric field is produced by a A constant magnetic field. B C changing magnetic field. D 4 Slide 20 / 99 either a constant or a changing magnetic field. none of the given answers A changing electric field will produce a A current. B C gravitational field. D none of the given answers magnetic field. Slide 21 / 99 Electromagnetic Waves Slide 22 / 99 The electric and magnetic waves are perpendicular to each other, and to the direction of propagation. Light is an Electromagnetic Wave Slide 23 / 99 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 Light is an Electromagnetic Wave Slide 24 / 99 Electromagnetic Radiation Slide 25 / 99 · All electromagnetic radiation travels at the same velocity: the speed of light (c), c = 3.00 ´ 108 m/s. · For all waves, velocity = wavelength x frequency: v = #f · Therefore for light, c = lf 5 All electromagnetic waves travel through a vacuum at A the same speed. B speeds that are proportional to their frequency. C speeds that are inversely proportional to their frequency. D none of the given answers 6 In a vacuum, the velocity of all electromagnetic waves A is zero. B C is 3.0 × 108 m/s. D depends on their amplitude. depends on the frequency. Slide 26 / 99 Slide 27 / 99 7 Of the following, which is not electromagnetic in nature? A microwaves B C sound waves D radio waves 8 gamma rays Which of the following correctly lists electromagnetic waves in order from longest to shortest wavelength? A gamma rays, ultraviolet, infrared, microwaves B microwaves, ultraviolet, visible light, gamma rays C radio waves, infrared, gamma rays, ultraviolet D television, infrared, visible light, X-rays 9 Slide 28 / 99 For a wave, the frequency times the wavelength is the wave's A speed. B C amplitude. D power. intensity. Slide 29 / 99 Slide 30 / 99 Slide 31 / 99 10 What color of light has the shortest wavelength? A Green B Red C Yellow D Blue 11 What color of light has the longest wavelength? A Green B Red C Yellow D Blue 12 Electromagnetic radiation travels through vacuum at a speed of A 186,000 m/s B 125 m/s C 3.00 x 10 m/s D It depends on wavelength 8 Slide 32 / 99 Slide 33 / 99 Slide 34 / 99 13 The wavelength of light that has a frequency of 1.20 x 1013s is A 25 m B 2.5 x 10 m C 0.040 m D 2.5 m -5 c = lf c = 3.00 ´ 108 m/s Slide 35 / 99 14 What is the frequency of light whose wavelength is 600 nm? A 5.0 x 10 14 B 1.0 x 10 15 Hz C 1.5 x 10 15 Hz D 2.0 x 10 15 Hz Hz c = lf c = 3.00 ´ 108 m/s 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 7.5 x 1014 Hz 500 nm 14 6 x 10 Hz 600 nm 14 5 x 10 Hz 700 nm 14 4 x 10 Hz 15 Visible light ranges in wavelength from A 400 μm to 750 μm. B C 400 nm to 750 nm. D 500 nm to 850 nm. 500 μm to 850 μm. 16 White light is A Slide 37 / 99 Slide 38 / 99 light of wavelength 550 nm, in the middle of the visible spectrum. B a mixture of all frequencies. C a mixture of red, green, and blue light. D the term used to describe very bright light. E the opposite (or complementary color) of black light. 17 Light with wavelength slightly longer than 750 nm is called A ultraviolet light. B C infrared light. D none of the given answers visible light. Slide 39 / 99 Interference – Young’s Double-Slit Experiment Slide 40 / 99 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 France s co Franco by Patrick Edwin Moran Waves Versus Particles: Huygens’ Principle Slide 41 / 99 Every point on a wave front acts as a point source; the wavefront as it develops is tangent to their envelope Diffraction When waves encounter an obstacle, they bend around it, leaving a “shadow region.” This is called diffraction. © Exploratorium, www.e xploratorium.e du. S ome rights re s e rve d. Unle s s othe rwis e note d, this work is lice ns e d unde r cre ative commons .org/lice ns e s /by-nc-s a/3.0/us / Slide 42 / 99 Diffraction Slide 43 / 99 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. 18 What principle is responsible for light spreading as it passes through a narrow slit? A refraction B C polarization diffraction D interference 19 What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? A refraction B C polarization D interference dispersion Slide 44 / 99 Slide 45 / 99 Slide 46 / 99 20 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 bright fringe B C gray fringe D multi-colored fringe dark fringe Double-Slit Maxima and Minima Slide 47 / 99 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 France s co Franco Double-Slit Maxima and Minima The bright lines that appear on the screen are called maxima. L x The dark lines are called minima. Maxima are evenly spaced, and a minimum occurs between each pair of maxima. d Extra distance = # The distance to the first maxima can be found by using similar triangles. Slide 48 / 99 Interference of Light Waves Slide 49 / 99 L bright spot bright spot θ1 θ2 d bright spot bright spot bright spot dark spot dark spot dark spot dark 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. Interference of Light Waves Slide 50 / 99 L bright spot d θ1 θ2 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 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. Slide 51 / 99 Double-Slit Maxima and Minima Slide 52 / 99 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 Interference - Young's Double Slit Experiment Slide 53 / 99 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. 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. Slide 54 / 99 Diffraction Grating Slide 55 / 99 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 Slide 56 / 99 21 What happens to a diffraction pattern if the wavelength of the light is decreased? A Interference fringes move closer to the central maximum. B Interference fringes move away from the central maximum. C There is no change in the interference. D Bright fringes are replanced with dark fringes. Slide 57 / 99 22 What happens to a diffraction pattern if the space between the slits is decreased? A Interference fringes move closer to the central maximum. B Interference fringes move away from the central maximum. C There is no change in the interference. D Bright fringes are replanced with dark fringes. 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 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 x# D m = 0, 1, 2, ... 1/2l L 1/2D x# lL D 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 Slide 60 / 99 Single Slit Interference Slide 61 / 99 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. Diffraction Interference Around an Object Slide 62 / 99 Light also bends around objects, creating a bright spot where it would be least expected. 23 What principle is responsible for alternating light and dark bands when light passes through two or more narrow slits? A refraction B C polarization dispersion D interference Slide 63 / 99 24 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 bright fringe B C gray fringe D multi-colored fringe Slide 64 / 99 dark fringe 25 The separation between adjacent maxima in a double-slit interference pattern using monochromatic light is A greatest for red light. B C greatest for green light. greatest for blue light. D the same for all colors of light. Slide 65 / 99 Slide 66 / 99 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: 26 Light travels fastest A in a vacuum. B C through water. D through diamond. Slide 67 / 99 through glass. 27 For all transparent material substances, the index of refraction A is less than 1. B C is greater than 1. D could be any of the given answers; it all depends on optical density. Slide 68 / 99 is equal to 1. 28 The index of refraction of diamond is 2.42. This means that a given type of light travels A 2.42 times faster in air than it does in diamond. B 2.42 times faster in diamond than it does in air. C 2.42 times faster in vacuum than it does in diamond. D 2.42 times faster in diamond than it does in vacuum. Slide 69 / 99 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. 29 When a light wave enters into a medium of different optical density, A B C D Slide 71 / 99 its speed and frequency change. its speed and wavelength change. its frequency and wavelength change. its speed, frequency, and wavelength change. 30 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 1.50. B C is reduced to 2/3 its original value. is unaffected. D none of the given answers Slide 72 / 99 31 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 1.50. B C is reduced to 2/3 its original value. is unaffected. D none of the given answers 32 When a light wave enters into a medium of different optical density, A B C D Slide 73 / 99 Slide 74 / 99 its speed and frequency change. its speed and wavelength change. its frequency and wavelength change. its speed, frequency, and wavelength change. 33 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 1.50. B C is reduced to 2/3 its original value. is unaffected. D none of the given answers Slide 75 / 99 34 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 1.50. B C is reduced to 2/3 its original value. is unaffected. D none of the given answers Dispersion Slide 76 / 99 Slide 77 / 99 The index of refraction of a material varies somewhat with the wavelength of the light. Dispersion This variation in refractive index is why a prism will split white light (which contains all the colors) into a rainbow of colors. Slide 78 / 99 35 White light is A Slide 79 / 99 light of wavelength 550 nm, in the middle of the visible spectrum. B a mixture of all frequencies. C a mixture of red, green, and blue light. D the term used to describe very bright light. E the opposite (or complementary color) of black light. 36 The principle which explains why a prism separates white light into different colors is A refraction. B C polarization. D total internal reflection. Slide 80 / 99 dispersion. 37 Which color of light undergoes the smallest refraction when passing from air to glass? A red B C yellow green D violet Slide 81 / 99 The Visible Spectrum and Dispersion Slide 82 / 99 Actual rainbows are created by dispersion in tiny drops of water. © Copyright RichTe a and lice ns e d for re us e unde r this Cre ative Commons Lice nce . © Copyright Be yonde r and lice ns e d for re us e unde r this Cre ative Commons Lice nce . 38 The principle which allows a rainbow to form is A refraction. B C polarization. D total internal reflection. Slide 83 / 99 dispersion. 39 Light with wavelength slightly shorter than 400 nm is called A ultraviolet light. B C infrared light. D none of the given answers visible light. Slide 84 / 99 Slide 85 / 99 40 Which color of light undergoes the greatest refraction when passing from air to glass? A red B C yellow green D violet Slide 86 / 99 Interference by Thin Films The colors on the soap bubble are created by interference by thin films. 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 C B Air Oil Water 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. 2t = (m+½) λ, where m = 1, 2, 3... A Air C B Oil Water nair < noil< nwater The wavelength, λ, is the wavelength in the film of oil and t is the thickness of the film. 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. 2t = mλ, where m = 1, 2, 3... A C Air Film B Air nair < nwater > nair The wavelength, λ, is the wavelength in the film of oil and t is the thickness of the film. 41 The colors on an oil slick are caused by reflection and A diffraction. B C interference. D polarization. refraction. Slide 90 / 99 Slide 91 / 99 42 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? Slide 92 / 99 43 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? Slide 93 / 99 Slide 94 / 99 Slide 95 / 99 44 Electromagnetic waves are A longitudinal. B C both longitudinal and transverse. D neither longitudinal or transverse. transverse. Slide 96 / 99 Polarization Slide 97 / 99 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 = I0 cos2 θ where θ is the angle between the polarizer axis and the plane of polarization and I0 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. 45 What principle is responsible for the fact that certain sunglasses can reduce glare from reflected surfaces? A refraction B C polarization diffraction D total internal reflection Slide 98 / 99 Slide 99 / 99 46 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 I0/4 B I0/4 C 3I0/8 D 3I0/4