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Physics 504 chapter 2 reflection of light

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  • 1. Physics 504Physics 504Reflections on LightReflections on Light
  • 2. Types of ReflectionTypes of Reflection• Specular (regular) reflection occurs whenSpecular (regular) reflection occurs whenthe reflected rays of light emerge parallelthe reflected rays of light emerge parallelfrom the surface.from the surface.• The surface is usually smooth andThe surface is usually smooth andpolished.polished.• E.g. mirror, glassE.g. mirror, glass
  • 3. Types of ReflectionTypes of Reflection• Diffuse reflection occurs when theDiffuse reflection occurs when thereflected rays of light are not parallel.reflected rays of light are not parallel.• The surface is usually rough and uneven.The surface is usually rough and uneven.• E.g. paper, snowE.g. paper, snow
  • 4. VocabularyVocabulary• Reflection in a plane mirror,Reflection in a plane mirror,• Incident ray – ray coming into surfaceIncident ray – ray coming into surface• Point of incidence – where the ray hits thePoint of incidence – where the ray hits thesurfacesurface• Reflected ray – the reflected rayReflected ray – the reflected ray• Normal – line at 90Normal – line at 90 ⁰⁰ to point of incidence.to point of incidence.
  • 5. Mirrors:Mirrors:• Mirrors are used in many ways becauseMirrors are used in many ways becausethey reflect light so well.they reflect light so well.• E.g. cameras (non-digital), periscopes,E.g. cameras (non-digital), periscopes,reflecting telescopes, solar heatersreflecting telescopes, solar heaters• P. 43, Q. 3-5P. 43, Q. 3-5
  • 6. Angle of Incidence & ReflectionAngle of Incidence & Reflection• The angles of incidence and reflection areThe angles of incidence and reflection aremeasured relative to the NORMAL –measured relative to the NORMAL –• NOT THE SURFACE.NOT THE SURFACE.• The angle of incidence equals the angle ofThe angle of incidence equals the angle ofreflection.reflection.• The incident ray, the reflected ray and theThe incident ray, the reflected ray and thenormal are all in the same plane.normal are all in the same plane.
  • 7. RaysRaysIncident Rays Reflected RaysIncident Rays Reflected Raysnormal normalAngle ofincidenceAngle ofreflection
  • 8. Formation of Image by a PlaneFormation of Image by a PlaneMirror – Ray DiagramMirror – Ray Diagram• A ray of light incident on a plane mirror at 90A ray of light incident on a plane mirror at 90oogetsgetsreflected from the mirror along the same path.reflected from the mirror along the same path.• A ray of light falling on a plane mirror at any angle getsA ray of light falling on a plane mirror at any angle getsreflected from the mirror such that the angle of incidencereflected from the mirror such that the angle of incidenceis equal to the angle of reflection.is equal to the angle of reflection.• P. 45, Q. 1, 3P. 45, Q. 1, 3
  • 9. Images formed in a Plane MirrorImages formed in a Plane Mirror• A plane mirror is a flat mirror.A plane mirror is a flat mirror.• Images are the same attitude – upright orImages are the same attitude – upright orinverted.inverted.• Images are the same distance “behind”Images are the same distance “behind”the mirror as the object is in front of it.the mirror as the object is in front of it.• Images formed are virtual – they cannotImages formed are virtual – they cannotbe projected on a screen.be projected on a screen.• A line joining the image and object isA line joining the image and object isperpendicular to the plane mirror.perpendicular to the plane mirror.
  • 10. QuestionQuestionAn object is placed in front of a plane mirror.Which of the following diagrams correctly represents the image formed in the plane mirror?A) C)B) D)
  • 11. QuestionQuestionAn object is placed in front of a plane mirror. Which statement correctly describes thecharacteristics of the image?A) The image is real, upright, smaller than the object and located in front of the mirror.B) The image is virtual, inverted, larger than the object and located behind the mirror.C) The image is real, inverted, the same size as the object and located in front of themirror.D) The image is virtual, upright, the same size as the object and located behind themirror.
  • 12. ExerciseExercise• Using a light beam, ruler and plane mirror,Using a light beam, ruler and plane mirror,draw the incident, normal and reflected raydraw the incident, normal and reflected raycoming off the mirror.coming off the mirror.• Compare the angles.Compare the angles.• ____________________________________________________________________
  • 13. Field of VisionField of Vision• The field of vision from a mirror isThe field of vision from a mirror isdetermined by the width of the mirror, thedetermined by the width of the mirror, thedistance of the observer to the mirror anddistance of the observer to the mirror andits curvature.its curvature.• In stores curved mirrors are used to give aIn stores curved mirrors are used to give awider field of vision.wider field of vision.
  • 14. FIELD OF VISIONFIELD OF VISIONnormal normalField of VisionObserverΘrΘiΘrΘiPlane mirror
  • 15. Field of VisionField of Vision• Our view in a mirror depends on how we positionourselves in front or to the side of the mirror.• The field of vision can be determined through raydiagrams:• a. Draw two normals, one at either end of the mirror,• b. Draw an incident ray from your eye to each normal.• c. Draw a reflected ray from each normal.• d. Remember the laws of reflection when drawing bothsets of rays (i = r)• Make a diagram – ask teacher.
  • 16. Activity: Determine Field of Vision:Activity: Determine Field of Vision:for a Plane Mirrorfor a Plane Mirror________MirrorMirror__________OO
  • 17. Activity: Determine Field of Vision:Activity: Determine Field of Vision:For a Curved MirrorFor a Curved MirrorOO
  • 18. QuestionQuestionAn observer is standing in front of a reflective window in which he can see the images of someshrubs.Based on the above diagram, the images of which shrubs can be seen by the observer?A) III onlyB) II and III onlyC) III and IV onlyD) I, II, III and IV
  • 19. ActivityActivity• P. 47, Q. 1-3P. 47, Q. 1-3
  • 20. 2020Concave MirrorConcave Mirror
  • 21. 2121The Inside of A SpoonThe Inside of A Spoon• Why is it when you look into a soup spoon thatWhy is it when you look into a soup spoon thatyour image turns upside down sometimes?your image turns upside down sometimes?• The inside of a spoon is a concave mirror.The inside of a spoon is a concave mirror.• If the inside of the spoon is held close to theIf the inside of the spoon is held close to theeye, a magnified upright view of the eye will beeye, a magnified upright view of the eye will beseen (in this case the eye is closer than the focalseen (in this case the eye is closer than the focalpoint of the mirror).point of the mirror).• If the spoon is moved farther away, a smallerIf the spoon is moved farther away, a smallerupside-down view of the whole face will be seen.upside-down view of the whole face will be seen.
  • 22. 2222Concave MirrorsConcave Mirrors• The concave mirror has a reflectionThe concave mirror has a reflectionsurface that curves inward, like a portionsurface that curves inward, like a portionof the interior of a sphere.of the interior of a sphere.• When light rays that are parallel to theWhen light rays that are parallel to theprincipal or optical axis reflect from theprincipal or optical axis reflect from thesurface of a concave mirror, theysurface of a concave mirror, theyconverge on the focal point (black dot) inconverge on the focal point (black dot) infront of the mirror.front of the mirror.
  • 23. 2323Converging MirrorConverging Mirror• Concave mirrors bring light rays to a focusConcave mirrors bring light rays to a focusand are called converging mirrors.and are called converging mirrors.
  • 24. 2424Rules for Con/Di-VergingRules for Con/Di-VergingMirrorsMirrors• A ray that is parallel to the principal axis isA ray that is parallel to the principal axis isreflected through the principal focus – real orreflected through the principal focus – real orvirtual.virtual.• A ray passing through the principal focus isA ray passing through the principal focus isreflected parallel to the principal axis.reflected parallel to the principal axis.• A ray passing through the centre of curvature isA ray passing through the centre of curvature isreflected back along the same path.reflected back along the same path.• The centre of curvature or radius is twice theThe centre of curvature or radius is twice thefocal length.focal length.• C = 2f.C = 2f.
  • 25. Exam Question using the 3RsExam Question using the 3RsLook at the diagram below. A 12 cm tall object is placed in front of a concave mirror. The focallength is 30 cm. The object is located 70 cm from the top of the mirror.What will be the height of the image reflected by the mirror?A) 5.1 cmB) 9.0 cmC) 16 cmD) 28 cm
  • 26. Concave MirrorsConcave MirrorsObject’sPositionImage CharacteristicsType Orientation Size PositionAt ∞ Real Point Image At FBeyond C Real Inverted Smaller Between F and CAt C Real Inverted Same size At CBetweenC and VReal Inverted Bigger Beyond CAt F No ImageBetweenF and VVirtual Upright Larger thanthe objectBetween F and V behind themirror, farther from themirror than the object.
  • 27. 2727Convex MirrorsConvex Mirrors• The convex mirror has a reflecting surface thatThe convex mirror has a reflecting surface thatcurves outward like a portion of the exterior of acurves outward like a portion of the exterior of asphere.sphere.• Light rays parallel to the optical axis areLight rays parallel to the optical axis arereflected from the surface in a manner thatreflected from the surface in a manner thatdiverges from the focal point, which is behind thediverges from the focal point, which is behind themirror.mirror.• Images formed with convex mirrors are alwaysImages formed with convex mirrors are alwaysright side up and reduced in size.right side up and reduced in size.• These images are also termed virtual images,These images are also termed virtual images,because they occur where reflected rays appearbecause they occur where reflected rays appearto diverge from a focal point behind the mirror.to diverge from a focal point behind the mirror.
  • 28. 2828Diverging MirrorDiverging Mirror• Diverging mirrors are convex.Diverging mirrors are convex.• Regardless of the position of the object reflectedRegardless of the position of the object reflectedby a convex mirror, the image formed is alwaysby a convex mirror, the image formed is alwaysvirtual, upright, and reduced in size.virtual, upright, and reduced in size.• When the brain retraces the rays they appear toWhen the brain retraces the rays they appear tocome from behind the mirror where they wouldcome from behind the mirror where they wouldconverge, producing a smaller upright imageconverge, producing a smaller upright image• The image is upright since the virtual image isThe image is upright since the virtual image isformed before the rays have crossed the focalformed before the rays have crossed the focalpoint.point.
  • 29. Convex MirrorsConvex MirrorsObject’sPositionImage CharacteristicsType Orientation Size PositionAnypositionVirtual Upright Smallerthan theobjectBetween F and V behind themirror, closer to the mirrorthan the object.
  • 30. Exam Question – on boardExam Question – on boardA) + 1.60B) + 0.63C) - 0.63D) - 1.63The teacher will draw this on the board, please.Alex places a 5.00 cm tall candle 30.0 cm in front of amirror that has a focal length of ‑50.0 cm.What is the magnification of the candle?
  • 31. ActivityActivity• Page 54, Q. 4, 7, 9Page 54, Q. 4, 7, 9
  • 32. Mirror EquationsMirror Equations• ddoo is the distance to the objectis the distance to the object• ddii is the distance to the imageis the distance to the image• f is the focal lengthf is the focal length• hhii is the image heightis the image height• hhoo is the object heightis the object height• N.B. the negative signN.B. the negative sign
  • 33. Conventions for the EquationsConventions for the Equations• Distances are measured from the vertex.Distances are measured from the vertex.• Focal lengths are positive for converging mirrorsFocal lengths are positive for converging mirrorsand negative for diverging mirrors.and negative for diverging mirrors.• Radii of curvature are positive for convergingRadii of curvature are positive for convergingmirrors and negative for d.m.mirrors and negative for d.m.• Image and object distances are positive for realImage and object distances are positive for realimages and objects.images and objects.• Image and object distances are negative forImage and object distances are negative forvirtual images and objects.virtual images and objects.• Image and object heights are positive whenImage and object heights are positive whenupright and negative when inverted.upright and negative when inverted.
  • 34. Exam QuestionExam Question A) 20.0 cm C) 90.0 cm B) 60.0 cm D) 120.0 cmAn object, 45.0 cm high, is placed in front of a convex mirror.A virtual image, 15.0 cm high, is formed.The focal length of the mirror is -30.0 cm.At what distance is the object from the mirror?
  • 35. ActivityActivity• Page 70, Q. 1-4Page 70, Q. 1-4• Page 76, Q. 1,2Page 76, Q. 1,2
  • 36. 3636Uses for Concave MirrorsUses for Concave Mirrors• Concave mirrors are used in optical telescopesConcave mirrors are used in optical telescopesto collect the faint light emitted from very distantto collect the faint light emitted from very distantstars.stars.• The curved surface concentrates parallel raysThe curved surface concentrates parallel raysfrom a great distance into a single point forfrom a great distance into a single point forenhanced intensity.enhanced intensity.• This mirror design is also commonly found inThis mirror design is also commonly found inshaving or cosmetic mirrors where the reflectedshaving or cosmetic mirrors where the reflectedlight produces a magnified image of the face.light produces a magnified image of the face.
  • 37. 3737Uses for Convex MirrorsUses for Convex Mirrors• Convex mirrors are often used in automobileConvex mirrors are often used in automobileright-hand rear-view applications where theright-hand rear-view applications where theoutward mirror curvature produces a smaller,outward mirror curvature produces a smaller,more panoramic view of events occurring behindmore panoramic view of events occurring behindthe vehicle.the vehicle.• Convex mirrors are also used as wide-angleConvex mirrors are also used as wide-anglemirrors in hallways and businesses for securitymirrors in hallways and businesses for securityand safety.and safety.• Carnival Fun House mirrors often incorporate aCarnival Fun House mirrors often incorporate amixture of concave and convex surfaces, ormixture of concave and convex surfaces, orsurfaces that gently change curvature, tosurfaces that gently change curvature, toproduce bizarre, distorted reflections whenproduce bizarre, distorted reflections whenpeople observe themselves.people observe themselves.
  • 38. Other ReflectionsOther Reflections• Non-optical reflection can occur as:Non-optical reflection can occur as:• Echoes – sounds reflected from surface;Echoes – sounds reflected from surface;• Radio and microwaves;Radio and microwaves;• Ocean waves when they recede;Ocean waves when they recede;• Strings/Springs – as in guitar strings;Strings/Springs – as in guitar strings;
  • 39. SummarySummary• A normal is a line drawn at right angles toA normal is a line drawn at right angles toa reflective surface at the point ofa reflective surface at the point ofincidence.incidence.• The Laws of Reflection are:The Laws of Reflection are:– The incident ray, the reflected ray & theThe incident ray, the reflected ray & thenormal are all in the same plane;normal are all in the same plane;– The angle of incidence equals the angle ofThe angle of incidence equals the angle ofreflectionreflection
  • 40. • Specular reflection is the reflection of lightSpecular reflection is the reflection of lightfrom smooth plane surfaces. Rays parallel.from smooth plane surfaces. Rays parallel.• Diffuse reflection is the reflection of lightDiffuse reflection is the reflection of lightfrom irregular surfaces. Rays not parallel.from irregular surfaces. Rays not parallel.• The field of vision using a plane mirrorThe field of vision using a plane mirrordepends on the size of the mirror and thedepends on the size of the mirror and thedistance of the observer and the object-to-distance of the observer and the object-to-be-seen from the mirror.be-seen from the mirror.• Curved mirror terms – centre of curvature,Curved mirror terms – centre of curvature,principal axis, vertex, radius of curvature,principal axis, vertex, radius of curvature,principal focus, focal plane & focal length.principal focus, focal plane & focal length.
  • 41. • All rays parallel to each other and strikingAll rays parallel to each other and strikinga converging mirror meet at the focala converging mirror meet at the focalpoint.point.• The focal length is the distance betweenThe focal length is the distance betweenthe principal focus and the vertex.the principal focus and the vertex.• In a curved mirror, the focal length is ½In a curved mirror, the focal length is ½the radius.the radius.• Rays from distant objects are consideredRays from distant objects are consideredparallel.parallel.
  • 42. • Spherical aberration occurs in curvedSpherical aberration occurs in curvedmirrors when parallel rays do not meet atmirrors when parallel rays do not meet atthe focal point. A parabola can solve this.the focal point. A parabola can solve this.• The Laws of Reflection apply to non-The Laws of Reflection apply to non-optical phenomena, like sound.optical phenomena, like sound.