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Does a changing Electric Field produce a magnetic field? The Replicating Wave 1 2 Maxwell’s equations basically say that a changing magnetic field produces a changing electric field which in turn produces a changing magnetic field and so on. Once you get this going these two kinds of fields would just continue replicating themselves, moving thru empty space where both J and are 0 , he then calculated the speed at which these would propagate to be the know speed of light c. Implication is that light must just be an electromagnetic wave. First Transatlantic communication in 1901 3 The Nature of Light: What causes Electric and Magnetic Fields? 4 C= 3 x 108 m/s Shake a charged rod and you generate electromagnetic waves. Shake slowly and you generate radio waves, shake quickly and you generate ( a million times per sec) and you can see them. 5 Wiggling one charge causes the field lines attached to it to wiggle, and after a time the other charge starts to wiggle! 6 1 Electromagnetic Waves 7 Electromagnetic Waves are Transverse Waves 8 Polarization • The and fields are perpendicular to each other • Both fields are perpendicular to the direction of motion – Therefore, em waves are transverse waves Demo: Polaroid 9 Charges and Fields, Summary 10 Electromagnetic Spectrum • Stationary charges produce only electric fields • Charges in uniform motion (constant velocity) produce electric and magnetic fields • Charges that are accelerated (or oscillate) produce electric and magnetic fields and electromagnetic waves 11 12 2 The EM Spectrum 13 14 How are EM waves produced? Is it correct to say that in every case, without exception, any radio wave travels faster than any sound wave? In glass, Red is scattered the most, violet the least 1. Yes 2. No 15 16 Check yourself 1.Is it correct to say that a radio wave is a low-frequency light wave? 2. Is a radio wave also a sound wave? 3.What is the wavelength of TV station broadcasting at 100 MGHertz? Just as sound can force a sound receiver into vibration, alight wave can force electrons in materials into vibration. For visible light this high frequency will only affect electrons (which basically have no mass) in the material. 17 18 3 Speed of Light: Olaus Roemer1675 Earth Io Sun Jupiter Period of Io is 42.5 hr. period was shorter by 1000 sec when earth was moving forward than when it was moving away. Extra Distance= 300,000,000,000 m c= Extra Time 1000 sec 19 Transparent materials A wave of visible light incident upon a pane of glass sets up in atoms' vibrations that produce a chain of absorptions and re-emissions, which pass the light energy through the material and out the other side. Because of the time delay between absorptions and reemissions, the light travels through the glass more slowly than through empty space. 20 Light and transparent materials At lower wave frequencies, such as those of visible light, electrons in the glass atoms are forced into vibration, but at less amplitude. The atoms hold the energy for less time, with less chance of collision with neighboring atoms, and less energy transformed to heat. The energy of vibrating electrons is re-emitted as light. Glass is transparent to all the frequencies of visible light. The frequency of the re-emitted light that is passed from atom to atom is identical to the frequency of the light that produced the vibration in the first place. However, there is a slight time delay between absorption and re-emission. 21 22 Only energy having the frequency of blue light is transmitted; energy of the other frequencies is absorbed and warms the glass. Check yourself 1. Why is glass transparent to visible light but opaque to ultraviolet and infrared? 23 24 4 Check yourself 1. When red light shines on a red rose, why do the leaves become warmer than the petals? Dispersion 2. When green light shines on a rose, why do the petals look black? Usually a material absorbs light of some frequencies and reflects the rest. If a material absorbs most of the visible light that is incident upon it but reflects red, for example, it appears red. That's why the petals of a red rose are red and the stem green. The atoms of the petals absorb all visible light except red, which they reflect; the stem absorbs all except green, which it reflects. An object that reflects light of all the visible frequencies, as the white part of this page does, is the same color as the light that shines on it. If a material absorbs all the light that shines on it, it reflects none and is black. 25 26 White Light White light from the sun is a composite of all the visible frequencies is easily demonstrated by passing sunlight through a prism and observing the rainbow-colored spectrum. The intensity of light from the sun varies with frequency, being most intense in the yellow-green part of the spectrum. The radiation curve of sunlight is a graph of brightness versus frequency. Sunlight is brightest in the yellow-green region, in the middle of the visible range. 27 Reflection of Light 28 Mixing Colors Most of the objects around us reflect rather than emit light. They reflect only part of the light that is incident upon them, the part that gives them their color 29 30 5 Why is the sky blue The tinier the particle, the greater amount of higherfrequency light it will re-emit Why sunsets are red 31 A sunbeam must travel through more atmosphere at sunset than at noon. As a result, more blue is scattered from the beam at sunset than at noon. By the time a beam of initially white light gets to the ground, only light of the lower frequencies survives to produce a red 33 sunset. The spaceship at S wishes to touch the surface of the giant planet and proceed to point X in the shortest distance possible. To what point P on the planet surface should the spaceship travel? A beam of light falls on an atom and increases the vibrational motion of electrons in the atom. The vibrating electrons re-emit the light in various directions. Light is scattered. Of the visible frequencies of sunlight, violet is scattered the most by nitrogen and oxygen in the atmosphere, followed by blue, green, yellow, orange, and red, in that order. Red is scattered only a tenth as much as violet. Although violet light is scattered more than blue, our eyes are not very sensitive to violet light. Therefore the blue scattered light is what predominates in our vision, and we see a blue sky! 32 Fermat’s Principal This idea was formulated by the French scientist Pierre Fermat in about 1650, and it is called Fermat's principle of least time. His idea is this: Out of all possible paths that light might take to get from one point to another, it takes the path that requires the shortest time . 34 Reflection 1. Point a 2. Point b 3. Point c 4. Actually, all yield the same total distance. 35 36 6 Why? Paper 37 38 Law of reflection 39 40 Plane Mirrors 41 42 7 In order that you are able to see a full-length view of yourself, the minimum size for a plane mirror must be In order that you are able to see a full-length view of yourself, the minimum size for a plane mirror must be 1. one-quarter your height. 2. one-half your height. 3. three-quarters your height. 4. your full height. 5. … depends on your distance. 1. one-quarter your height. 2. one-half your height. 3. three-quarters your height. 4. your full height. 5. … depends on your distance. 43 44 45 46 Curved Mirrors Curved Mirrors a ) The virtual image formed by a convex mirror (a mirror that curves outward) is smaller and closer to the mirror than the object. ( b ) When the object is close to a concave mirror (a mirror that curves inward like a “cave”), the virtual image is larger and farther away than the object. In either case the law of reflection holds for each ray. 47 Curved Mirror 48 8 Object Located Beyond the Center of Curvature Focal Length Shown by Parallel Rays •A parallel ray goes thru focal point after hitting miror 49 •A ray thru focal point emerges parallel 50 Object Located Between the Center of Curvature and the Focal Point Object Located At the Center of Curvature 51 Object Located Between the Focal Point and the Mirror 53 52 Curved Mirrors 54 9 55 56 57 58 Why we see the penny A Straw in water looks bent Demo: Penny in bowl 59 60 10 Refraction of Light Why light bends 61 Why Light Bends Demo Note that when the pulse reaches the end of the medium, a portion of its energy is transmitted into the more dense medium (in the form of a transmitted pulse), a portion of its energy remains in the less dense medium (in the form of a reflected pulse). Second, observe that the transmitted pulse has a smaller speed and a smaller wavelength than the 62 incident pulse. Fermat’s Principal 63 Refraction 64 Snell’s Law Imagine that you are a lifeguard at a beach and you spot a person in distress in the water. We show the relative positions of you, the shoreline, and the person in distress in Figure 28.13 . You are at point A, and the person is at point B. You can run faster than you can swim. Should you travel in a straight line to get to B? The angle of incidence is larger than the angle of refraction by an amount that depends on the relative speeds of light in air and in water. 65 66 11 Check Yourself Suppose our lifeguard in the preceding example were a seal instead of a human being. How would its path of least time from A to B differ? Check your answer The seal can swim faster than it can run, and its path would bend as shown; likewise with light emerging from the bottom of a piece of glass into air. 67 68 Index of Refraction Speed of light in vacuum n= Speed of light in medium n=index of refraction For most transparent materials n is between 1-2 69 70 Light rays bend as they pass from air into water at a non–90 degree angle. This is refraction. Which quantity doesn’t change when light refracts? Light rays bend as they pass from air into water at a non–90 degree angle. This is refraction. Which quantity doesn’t change when light refracts? 1. Average speed of light 2. Material’s index of refraction 3. Frequency of light 4. Wavelength of light 1. Average speed of light 2. Material’s index of refraction 3. Frequency of light 4. Wavelength of light 71 72 12 Physics of Rainbow 73 When your eye is located between the sun (not shown off to the left) and a water-drop region, the rainbow you see is the edge of a threedimensional cone that extends through the water-drop region. 74 When light slows down in going from one medium to another, like going from air to water, it refracts toward the normal. When it speeds up in traveling from one medium to another, like going from water to air, it refracts away from the normal. 75 Refraction through glass. Although dashed line AC is the shortest path, light goes a slightly longer path through the air from A to a, than a shorter path through the glass to c, and then to C. The emerging light is displaced but parallel to the incident light. 77 76 Atmospheric Refraction Because of atmospheric refraction, when the sun is near the horizon it appears higher in the sky 78 13 Mirage Light from the sky picks up speed in air near the ground because that air is warmer and less dense than the air above. When the light grazes the surface and bends upward, the observer sees a mirage. 79 80 Fish in Water 81 Jose wishes to “spear” a fish with a laser. In order to make a direct hit, he should aim the laser beam 82 Jose wishes to “spear” a fish with a laser. In order to make a direct hit, he should aim the laser beam 1. above 2. below 3. directly at the observed fish. 1. above 2. below 3. directly at the observed fish. 83 84 14 Critical Angle 85 86 Total Internal Reflection Light emitted in the water is partly refracted and partly reflected at the surface. The blue dashes show the direction of light and the length of the arrows indicates the proportions refracted and reflected. Beyond the critical angle the beam is totally internally reflected. 87 An observer under water sees a circle of light at the still surface. Beyond a cone of 96。 (twice the critical angle) an observer sees a reflection of the water interior or bottom. 88 Fiber Optics Total internal reflection occurs in glass surrounded by air, for the speed of light in glass is less than in air. The critical angle for glass is about 43。, depending on the type of glass. So light in the glass that is incident at angles greater than 43。 to the surface is totally internally reflected. No light escapes beyond this angle; instead, all of it is reflected back into the glass 89 90 15 Focal Length of a Converging Lens Focal Length of a Diverging Lens • The parallel rays pass through the lens and converge at the focal point • The parallel rays can come from the left or right of the lens • The parallel rays diverge after passing through the diverging lens • The focal point is the point where the rays appear to have originated 91 92 93 94 95 96 Diverging lens •A parallel ray, when extrapolated backwards goes thru fp •A ray heading at fp point on other side, comes out parallel •A ray going toward C, goes undeflected 16 Optical Bench 97 The Eye 98 The Eye • The normal eye focuses light and produces a sharp image • Essential parts of the eye – Cornea – light passes through this transparent structure – Aqueous Humor – clear liquid behind the cornea 99 100 101 102 17 103 104 105 106 107 108 18 The Eye – Near Point Far Point • distance farthest from the eye at which an object can be clearly focused is called the far point. If your far point is not very far away, you are nearsighted and you need corrective lenses. • The near point is the closest distance for which the lens can accommodate to focus light on the retina – Typically at age 10, this is about 18 cm – It increases with age 109 Conditions of the Eye 110 Farsightedness • Eyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eye • Eyes may be – Farsighted • Light rays reach the retina before they converge to form an image – Nearsighted • Also called hyperopia • The image focuses behind the retina • Can usually see far away objects clearly, but not nearby objects • Person can focus on nearby objects but not those far away 111 Correcting Farsightedness 112 Compound Microscope • A converging lens placed in front of the eye can correct the condition • The lens refracts the incoming rays more toward the principle axis before entering the eye – This allows the rays to converge and focus on the retina 113 114 19 Telescopes Refracting Telescope • • Two fundamental types of telescopes – Refracting telescope uses a combination of lenses to form an image • – Reflecting telescope uses a curved mirror and a lens to form an image • • Telescopes can be analyzed by considering them to be two optical elements in a row – The image of the first element becomes the object of the second element • The two lenses are arranged so that the objective forms a real, inverted image of a distant object The image is near the focal point of the eyepiece The two lenses are separated by the distance ƒ o + ƒ e which corresponds to the length of the tube The eyepiece forms an enlarged, inverted image of the first image 115 Why we think light is a wave: Diffraction 116 Two Slit Interference Amount of bending depends on wavelength and slit width 117 118 119 120 20 Spectrum The wavelength of laser light δ = r2 – r1 = d tan θ = d y/L 121 122 123 124 125 126 21 Spectra of Helium Gas 127 Absorption Lines 128 Doppler Shift for light 129 130 The Hubble Graph The Age of the Universe 131 132 22