Last Time…Light rays and imagesQuestionHow do you ‘see’ this?Making a real imageHow a lens worksPowerPoint PresentationDifferent object positionsQuick QuizMaking an imageYour eye can change focal lengthImage size vs object size: MagnificationSlide 13Slide 14Virtual imagesSlide 16Magnifying glassSlide 18Lens combinationsCompound MicroscopeFar away objectsTelescope: object far awayNearsightednessFixing nearsightednessLast Time… Reflection and refractionTotal internal reflectionMon. Feb. 4, 2008 Physics 208, Lecture 4 2Light rays and imagesEach point on object reflects lightLight propagates out, represented by rays perpendicular to wavefront.Lens in our eye does some ‘imaging’ so that we identify origin of light rays.Mon. Feb. 4, 2008 Physics 208, Lecture 4 3QuestionDoes the fish appearA. Closer than actualB. Farther than actualC. Same as actualn=1.00n=1.33Virtual imageMon. Feb. 4, 2008 Physics 208, Lecture 4 4LensObjectImageHow do you ‘see’ this?Lens bends (refracts) light rays— forms image on retinaSensors on retina report to brain.Color information, intensity informationMon. Feb. 4, 2008 Physics 208, Lecture 4 5Making a real imageLens: Refracts light so that rays originating from a point are focused to a point on the other side.This is a real imageMon. Feb. 4, 2008 Physics 208, Lecture 4 6Fn1n2>n1n1How a lens worksPosition surfaces to bend light rays in just the right waySpherical surfaces are very close to the right ones.Optical AxisMon. Feb. 4, 2008 Physics 208, Lecture 4 71) Rays parallel to optical axis pass through focal point.2) Rays through center of lens are not refracted.3) Rays through F emerge parallel to optical axis.Thin-lens approximation: Ray tracingFFObjectImageOptical AxisHere image is real, inverted, enlargedMon. Feb. 4, 2008 Physics 208, Lecture 4 8Different object positionsImage (real, inverted)Image (real, inverted)ObjectImage (virtual, upright)These rays seem to originatefrom tip of a ‘virtual’ arrow.Mon. Feb. 4, 2008 Physics 208, Lecture 4 9Quick QuizI project a focused image onto a screen 2 meters away. I now want to make the image bigger without changing the lens. I shouldA. Move screen farther away onlyB. Move screen closer onlyC. Move screen closer and object toward lensD. Move screen farther and object toward lensE. Move screen farther and object away from lensMon. Feb. 4, 2008 Physics 208, Lecture 4 10Making an images’sObjectImageImage distanceObject distancefffocal lengthHow are all these related?€ 1s+1 ′s =1fMon. Feb. 4, 2008 Physics 208, Lecture 4 11LensObjectImage Your eye can change focal lengthWhat is range of focal lengths if it can focus from near point (25 cm) to inf. onto retina 1.7 cm away?€ 1s+1 ′s =1fObject at infinity: € 1∞+11.7cm=1f⇒ f = 1.7cmObject at near point: € 125cm+11.7cm=1f⇒ f = 1.59cmVery limited rangeMon. Feb. 4, 2008 Physics 208, Lecture 4 12Image size vs object size: Magnification= M = Magnification € image heightobject height= ′s s=image distanceobject distanceImage heightImage distanceObjectdist.ObjectheightMon. Feb. 4, 2008 Physics 208, Lecture 4 13QuestionAt what object distance does image size equal object size (magnification=1)?€ 1s+1 ′s =1fA. Object distance = fB. Object distance = 2fC. Object distance = f/2D. Object distance = infinityE. Object distance =0Mon. Feb. 4, 2008 Physics 208, Lecture 4 14Different object positionsImage (real, inverted)Image (real, inverted)ObjectImage (virtual, upright)These rays seem to originatefrom tip of a ‘virtual’ arrow.Mon. Feb. 4, 2008 Physics 208, Lecture 4 15Virtual imagesVirtual image can’t be recorded on film, Can’t be seen on a screen. But rays can be focused by another lense.g. lens in your eye (focus on retina)e.g. lens in a camera (focus on film plane)Image (virtual, upright)These rays seem to originatefrom tip of a ‘virtual’ arrow.objects closer to a converging lens than the focal length form a virtual imageMon. Feb. 4, 2008 Physics 208, Lecture 4 16Do these rays come from real image, a virtual image, or an object?Can’t tell. Rays are exactly equivalent, and can be imaged by a lens in exactly the same way.Mon. Feb. 4, 2008 Physics 208, Lecture 4 17Object at the near point - this is the biggest it will appear and be in focusMagnifying glasso•Object closer than focal point—Lens produces virtual image •Light rays appear to originate from virtual image•Virtual image is used as object for eye lens.•Have moved object ‘closer’, while permitting eye to focusImage (virtual, upright)pqMon. Feb. 4, 2008 Physics 208, Lecture 4 18Image (real, inverted)Image (virtual, upright)Normal magnifier config,virtual image, uprightImage (real, inverted)Image real, but eye cannot image itBlurryEye can now form imageof inverted lens imageMon. Feb. 4, 2008 Physics 208, Lecture 4 19Lens combinationsImage of one lens acts as object for next.Rays originate from image, whether real or virtualCan then directly apply€ 1s+1 ′s =1fMon. Feb. 4, 2008 Physics 208, Lecture 4 20Virtual image‘Object’EyepieceCompound MicroscopeqpObjectReal, inverted, imageObjectiveObject outside focal pointForms a real imageReal image used as object for eyepiece.Eyepiece forms virtual image for eye.Mon. Feb. 4, 2008 Physics 208, Lecture 4 21Far away objectsThe moon is 3.8x108 m away, and 3.5x106 m diameter. I use a 1 m focal length lens to make an image of the moon. About what diameter is this image of the moon?A. 0.5 cmB. 1 cmC. 2 cmD. 10 cmE. 1 mNot a very big image. How can It be made ‘bigger’?Mon. Feb. 4, 2008 Physics 208, Lecture 4 22Telescope: object far awayMon. Feb. 4, 2008 Physics 208, Lecture 4 23ObjectNearsightednessI can’t focus on thisThis, I can seeMon. Feb. 4, 2008 Physics 208, Lecture 4 24ObjectFixing
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