Image Formation with Concave Spherical Mirrors The figure shows a concave mirror, a mirror in which the edges curve toward the light source. Rays parallel to the optical axis reflect and pass through the focal point of the mirror. 4/28/2014 PHYS 132 1 A Spherical Mirror: Central Rays A few rays are easy to figure out where they go. All rays satisfy the “angle of incidence = angle of reflection” measured to the normal to the surface All rays through the center strike the mirror perpendicular to the surface and bounce back along their incoming path. center of sphere 4/28/2014 PHYS 132 2 A Spherical Mirror: Central Ray A few rays are easy to figure out where they go. All rays satisfy the “angle of incidence = angle of reflection” measured to the normal to the surface The ray hitting the central line of the diagram is particularly simple. center of sphere 4/28/2014 PHYS 132 3 A Spherical Mirror: Parallel Rays A few rays are easy to figure out where they go. All rays satisfy the “angle of incidence = angle of reflection” measured to the normal to the surface All rays parallel to and near an axis of the sphere reflect through a single point on the axis (the focal point) center of sphere 4/28/2014 PHYS 132 4 A Real Image Formed by a Concave Mirror 4/28/2014 PHYS 132 5 Images in a Spherical Mirror: 1 Physical center of sphere focal point 4/28/2014 PHYS 132 6 Tactics: Ray Tracing for a Spherical Mirror 4/28/2014 PHYS 132 7 4/28/2014 PHYS 132 8 The Mirror Equation For a spherical mirror with negligible thickness, the object and image distances are related by: where the focal length f is related to the mirror’s radius of curvature by: 4/28/2014 PHYS 132 9 You see an upright, magnified image of your face when you look into magnifying “cosmetic mirror.” The image is located 4/28/2014 PHYS 132 it. . be ca us e su rfa ce . yo ur m ir r or ’s On ly in m th e in d su rfa ce . m irr or ’s th e On Be hi nd fr on to ft he m ir r or ’s su rfa ce . 25% 25% 25% 25% In A. In front of the mirror’s surface. B. On the mirror’s surface. C. Behind the mirror’s surface. D. Only in your mind because it’s a virtual image. 10 Example 23.17 Analyzing a Concave Mirror 4/28/2014 PHYS 132 11 Example 23.17 Analyzing a Concave Mirror 4/28/2014 PHYS 132 12 Example 23.17 Analyzing a Concave Mirror 4/28/2014 PHYS 132 13 Example 23.17 Analyzing a Concave Mirror 4/28/2014 PHYS 132 14 Image Formation with Spherical Mirrors A city skyline is reflected in this polished sphere. 4/28/2014 PHYS 132 15 Image Formation with Convex Spherical Mirrors The figure shows parallel light rays approaching a mirror in which the edges curve away from the light source. This is called a convex mirror. The reflected rays appear to come from a point behind the mirror. 4/28/2014 PHYS 132 16 A Real Image Formed by a Convex Mirror 4/28/2014 PHYS 132 17 Lenses The photos below show parallel light rays entering two different lenses. The left lens, called a converging lens, causes the rays to refract toward the optical axis. The right lens, called a diverging lens, refracts parallel rays away from the optical axis. 4/28/2014 PHYS 132 18 Converging Lenses A converging lens is thicker in the center than at the edges. The focal length f is the distance from the lens at which rays parallel to the optical axis converge. The focal length is a property of the lens, independent of how the lens is used. 4/28/2014 PHYS 132 19 Diverging Lenses A diverging lens is thicker at the edges than in the center. The focal length f is the distance from the lens at which rays parallel to the optical axis appear to diverge. The focal length is a property of the lens, independent of how the lens is used. 4/28/2014 PHYS 132 20 You can use the sun’s rays and a lens to start a fire. To do so, you should use 4/28/2014 33% di v. .. or a Ei th er ac A di ve rg in g en s co nv er gi ng l A 33% le ns . . 33% on ve rg in g A. A converging lens. B. A diverging lens. C. Either a converging or a diverging lens will work if you use it correctly. PHYS 132 21 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin converging lens. Situation 1: A ray initially parallel to the optic axis will go through the far focal point after passing through the lens. 4/28/2014 PHYS 132 22 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin converging lens. Situation 2: A ray through the near focal point of a thin lens becomes parallel to the optic axis after passing through the lens. 4/28/2014 PHYS 132 23 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin converging lens. Situation 3: A ray through the center of a thin lens is neither bent nor displaced but travels in a straight line. 4/28/2014 PHYS 132 24 Thin Lenses: Ray Tracing Rays from an object point P are refracted by the lens and converge to a real image at point P′. 4/28/2014 PHYS 132 25 A lens produces a sharply focused, inverted image on a screen. What will you see on the screen if the lens is removed? 4/28/2014 PHYS 132 im ag e An in ve rte d bu tb lu rr yi th m at ag is e. di m A m sh er ar bu p, ... up r ig A ht bl im ur ry ag ,u e. pr ig ht im ag No e. im ag e at al l. 20% 20% 20% 20% 20% An A. An inverted but blurry image. B. An image that is dimmer but otherwise unchanged. C. A sharp, upright image. D. A blurry, upright image. E. No image at all. 26 A lens produces a sharply focused, inverted image on a screen. What will you see on the screen if a piece of dark paper is lowered to cover the top half of the lens? 4/28/2014 PHYS 132 im ag e An in ve rte d bu tb lu rr yi th m a t ag On i s e. ly di th m m e er to p bu On ha t.. ly lf . th of e th bo e im tto ag m e. ha lf of th e ... No im ag e at al l. 20% 20% 20% 20% 20% An A. An inverted but blurry image. B. An image that is dimmer but otherwise unchanged. C. Only the top half of the image. D. Only the bottom half of the image. E. No image at all. 27 A lens produces a sharply focused, inverted image on a screen. What will you see on the screen if the lens is covered by a dark mask having only a small hole in the center? 4/28/2014 PHYS 132 im ag e An in ve rte d bu tb lu rr yi th m a t ag On i s e. ly di th m m e er to p bu On ha t.. ly lf . th of e th bo e im tto ag m e. ha lf of th e ... No im ag e at al l. 20% 20% 20% 20% 20% An A. An inverted but blurry image. B. An image that is dimmer but otherwise unchanged. C. Only the top half of the image. D. Only the bottom half of the image. E. No image at all. 28 Image Formation The figure is a close-up view of the rays very near the image plane. To focus an image, you must either move the screen to coincide with the image plane or move the lens or object to make the image plane coincide with the screen. 4/28/2014 PHYS 132 29 Tactics: Ray Tracing for a Converging Lens 4/28/2014 PHYS 132 30 Tactics: Ray Tracing for a Converging Lens 4/28/2014 PHYS 132 31 A lens creates an image as shown. In this situation, the object distance s is PHYS 132 f. 33% en gt h le ng th f. f. en gt h th e Sh or te rt ha n Eq ua l to th e fo ca ll fo ca ll th e th an La rg er 4/28/2014 33% fo ca l A. Larger than the focal length f. B. Equal to the focal length f. C. Shorter than the focal length f. 33% 32 A lens creates an image as shown. In this situation, the image distance s′ is PHYS 132 f. 33% en gt h le ng th f. f. en gt h th e Sh or te rt ha n Eq ua l to th e fo ca ll fo ca ll th e th an La rg er 4/28/2014 33% fo ca l A. Larger than the focal length f. B. Equal to the focal length f. C. Shorter than the focal length f. 33% 33 Lateral Magnification The image can be either larger or smaller than the object, depending on the location and focal length of the lens. The lateral magnification m is defined as: A positive value of m indicates that the image is upright relative to the object. A negative value of m indicates that the image is inverted relative to the object. The absolute value of m gives the size ratio of the image and object: h′/h = |m|. 4/28/2014 PHYS 132 34 Virtual Images Consider a converging lens for which the object is inside the focal point, at distance s < f. You can see all three rays appear to diverge from point P′. Point P′ is an upright, virtual image of the object point P. 4/28/2014 PHYS 132 35 Virtual Images You can “see” a virtual image by looking through the lens. This is exactly what you do with a magnifying glass, microscope or binoculars. 4/28/2014 PHYS 132 36 Example 23.9 Magnifying a Flower 4/28/2014 PHYS 132 37 Example 23.9 Magnifying a Flower 4/28/2014 PHYS 132 38 Example 23.9 Magnifying a Flower 4/28/2014 PHYS 132 39 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin diverging lens. Situation 1: A ray initially parallel to the optic axis will appear to diverge from the near focal point after passing through the lens. 4/28/2014 PHYS 132 40 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin diverging lens. Situation 2: A ray directed along a line toward the far focal point becomes parallel to the optic axis after passing through the lens. 4/28/2014 PHYS 132 41 Thin Lenses: Ray Tracing Three situations form the basis for ray tracing through a thin diverging lens. Situation 3: A ray through the center of a thin lens is neither bent nor displaced but travels in a straight line. 4/28/2014 PHYS 132 42 Light rays are converging to point 1. The lens is inserted into the rays with its focal point at point 1. Which picture shows the rays leaving the lens? 4/28/2014 20% E 20% D 20% C 20% B A 20% PHYS 132 43 Tactics: Ray Tracing for a Diverging Lens 4/28/2014 PHYS 132 44 Example 23.10 Demagnifying a Flower 4/28/2014 PHYS 132 45 Example 23.10 Demagnifying a Flower 4/28/2014 PHYS 132 46 Example 23.10 Demagnifying a Flower 4/28/2014 PHYS 132 47 Wave Model 4/28/2014 PHYS 132 48 The second model for light: Electromagnetic wave • Light is an oscillating electromagnetic wave. (Long story) • A “close-up” of a ray: a plane wave 4/28/2014 PHYS 132 49 It’s hard to picture EM waves in 3D • Let’s build some intuition by working through a simpler example. Waves on the surface of water (treating the height of the surface only – that moves up and down – transvers to the wave motion: the actual bits of water move in small circles) 4/28/2014 PHYS 132 50 http://www.falstad.com/ripple/ Ripple tank analogy Can two sources lead to both “bright spots” and “dark spots”? 4/28/2014 PHYS 132 51 Chapter 22 Preview 4/28/2014 PHYS 132 52 100 micron slit Spot actually gets wider… Does this mean light has a “size”? 4/28/2014 PHYS 132 53 Chapter 22 Preview 4/28/2014 PHYS 132 54 What a difference a slit makes The big deal here is that opening an additional slit makes it darker in some places. No way this happens in either the ray or photon model. 4/28/2014 PHYS 132 55 Diffraction of Light When red light passes through an opening that is only 0.1 mm wide, it does spread out. Diffraction of light is observable if the hole is sufficiently small. 4/28/2014 PHYS 132 56 Young’s Double-Slit Experiment 4/28/2014 PHYS 132 57 Young’s Double-Slit Experiment 4/28/2014 PHYS 132 58 Analyzing Double-Slit Interference The figure shows the “big picture” of the double-slit experiment. The next slide zooms in on the area inside the circle. 4/28/2014 PHYS 132 59 Analyzing Double-Slit Interference The figure shows a magnified portion of the double-slit experiment. The wave from the lower slit travels an extra distance. Bright fringes (constructive interference) will occur at angles θm such that ∆r = mλ, where m = 0, 1, 2, 3, … 4/28/2014 PHYS 132 60 Analyzing Double-Slit Interference The mth bright fringe emerging from the double slit is at an angle: where θm is in radians, and we have used the smallangle approximation. The y-position on the screen of the mth bright fringe on a screen a distance L away is: 4/28/2014 PHYS 132 61 A laboratory experiment produces a double-slit interference pattern on a screen. The point on the screen marked with a dot is how much farther from the left slit than from the right slit? A. B. C. D. E. 1.0 λ. 1.5 λ. 2.0 λ. 2.5 λ. 3.0 λ. 4/28/2014 PHYS 132 62 A laboratory experiment produces a double-slit interference pattern on a screen. If the screen is moved farther away from the slits, the fringes will be A. closer together. B. in the same positions. C. farther apart. D. fuzzy and out of focus. 4/28/2014 PHYS 132 m e fa rt he ra pa fu rt. zz ya nd ou to ff oc us . in th e sa clo se rt og et he r. po si t io ns . 25% 25% 25% 25% 63 A laboratory experiment produces a double-slit interference pattern on a screen. If green light is used, with everything else the same, the bright fringes will be 4/28/2014 fa rt he ra pa rt. no fr in ge sb ec ... be ill w r. th e sa m e rt og et he po si t io ns . Th er e ∆y = 25% 25% 25% 25% in closer together. in the same positions. farther apart. There will be no fringes because the conditions for interference won’t be satisfied.λ L clo se A. B. C. D. and green light has a shorter wavelength. d PHYS 132 64 A laboratory experiment produces a double-slit interference pattern on a screen. If the slits are moved closer together, the bright fringes will be 4/28/2014 PHYS 132 fa rt he ra pa be rt. no fr in ge sb ec ... ill w r. sa m e rt og et he po si t io ns . Th er e d and d is smaller. th e ∆y = 25% 25% 25% 25% in closer together. in the same positions. farther apart. There will be no fringes because the conditions for interference won’t be satisfied. λL clo se A. B. C. D. 65 The figure shows what happens if you put white light through the same slit-screen system. Why are the different colors separated on either side of the center? 4/28/2014 PHYS 132 66 Intensity of the Double-Slit Interference Pattern The intensity of the double-slit interference pattern at position y is: 4/28/2014 PHYS 132 67 Intensity of the Double-Slit Interference Pattern The actual intensity from a double-slit experiment slowly decreases as |y| increases. 4/28/2014 PHYS 132 68 Single-Slit Diffraction Diffraction through a tall, narrow slit is known as single-slit diffraction. A viewing screen is placed distance L behind the slit of width a, and we will assume that L >> a. 4/28/2014 PHYS 132 69 Huygens’ Principle: Plane Waves 4/28/2014 PHYS 132 70 Huygens’ Principle: Spherical Waves 4/28/2014 PHYS 132 71 Analyzing Single-Slit Diffraction The figure shows a wave front passing through a narrow slit of width a. According to Huygens’ principle, each point on the wave front can be thought of as the source of a spherical wavelet. 4/28/2014 PHYS 132 72 Single-Slit Diffraction The light pattern from a single slit consists of a central maximum flanked by a series of weaker secondary maxima and dark fringes. The dark fringes occur at angles: 4/28/2014 PHYS 132 73 The Width of a Single-Slit Diffraction Pattern The central maximum of this single-slit diffraction pattern is much brighter than the secondary maximum. The width of the central maximum on a screen a distance L away is twice the spacing between the dark fringes on either side: The farther away from the screen (larger L), the wider the pattern of light becomes. The narrower the opening (smaller a), the wider the pattern of light becomes! 4/28/2014 PHYS 132 74 4/28/2014 PHYS 132 25% 25% D 25% C A 25% B A laboratory experiment produces a double-slit interference pattern on a screen. If the left slit is blocked, the screen will look like 75