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# Physics 504 chapter 4 lenses

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### Physics 504 chapter 4 lenses

1. 1. Physics 504Physics 504Chapter 4Chapter 4LensesLenses
2. 2. Types of LensesTypes of Lenses ConvergingConverging:: Causes light rays to come togetherCauses light rays to come together Examples: Magnifying glass, camera, eyeExamples: Magnifying glass, camera, eyeglassesglasses Diverging:Diverging: Causes light rays to spread apartCauses light rays to spread apart Examples: Microscope, telescope, eyeExamples: Microscope, telescope, eyeglassesglasses
3. 3. Principal RaysPrincipal Rays Just like curved mirrors, lenses are used toJust like curved mirrors, lenses are used toproduce imagesproduce images The images are found by using three principalThe images are found by using three principalrays:rays: Principal Ray 1:Principal Ray 1: A light ray parallel to the principalA light ray parallel to the principalaxis is refracted through the principal focal point.axis is refracted through the principal focal point. Principal Ray 2:Principal Ray 2: Passes through the optical center.Passes through the optical center.No apparent refraction occurs but the ray is bent.No apparent refraction occurs but the ray is bent.The ray seems to pass straight through to the otherThe ray seems to pass straight through to the otherside.side. Principal Ray 3:Principal Ray 3: Passes through the secondaryPasses through the secondaryfocal point and will be refracted. Will exit the lensfocal point and will be refracted. Will exit the lensparallel to the principal axis.parallel to the principal axis.
4. 4. Converging LensConverging Lens
5. 5. LensLens
6. 6. Diverging LensesDiverging Lenses Principal ray 1Principal ray 1 (parallel to the principal axis,(parallel to the principal axis,PA) will be refracted away from thePA) will be refracted away from the PAPA. But the. But theprolongation of the ray will pass through theprolongation of the ray will pass through theprincipal focus.principal focus. Principal ray 2Principal ray 2 (straight through the optical(straight through the opticalcenter) has no apparent sign of refraction.center) has no apparent sign of refraction. Principal ray 3Principal ray 3 (via the secondary focal point)(via the secondary focal point)is refracted parallel to the principal axis.is refracted parallel to the principal axis.
7. 7. Diverging LensDiverging Lens
8. 8. ActivityActivity Page 98, Q. 1-3Page 98, Q. 1-3
9. 9. Optical PowerOptical Power The strength of a lens (optical power) is relatedThe strength of a lens (optical power) is relatedto its focal length. A short focal length meansto its focal length. A short focal length meansthe light rays are being refracted a lot. Thethe light rays are being refracted a lot. Theoptical power of the lens is strong.optical power of the lens is strong. A long focal length means the light rays are notA long focal length means the light rays are notbeing refracted very much. The optical powerbeing refracted very much. The optical powerof the lens is weak.of the lens is weak. The power of a lens is equal to the inverse ofThe power of a lens is equal to the inverse ofthe focal point. P = 1/fthe focal point. P = 1/f Power unit: dioptres (d)Power unit: dioptres (d) Focal length: meters (m), negative if divergingFocal length: meters (m), negative if diverging
10. 10. Lens CombinationsLens Combinations By combining lenses, different optical powers can beBy combining lenses, different optical powers can beobtained. This is quite useful for making telescopes,obtained. This is quite useful for making telescopes,microscopes or any other optical instrument that usesmicroscopes or any other optical instrument that usesmore than one lens.more than one lens. TheThe optical powersoptical powers can be added using the equation:can be added using the equation: PPTT = P= P11 + P+ P22 + P+ Pnn…… PPTT: Total optical power in dioptres: Total optical power in dioptres PP11, P, P22, P, Pnn,: Power of each lens in dioptres.,: Power of each lens in dioptres. N.B.N.B. The focal lengthsThe focal lengths CAN NOTCAN NOT be added together tobe added together tosolve optical power. You must use the equationsolve optical power. You must use the equation P=1/fP=1/fto obtain theto obtain the PP value.value.
11. 11. Lens CombinationsLens Combinations What is the power, focal lengths?What is the power, focal lengths?
12. 12. ActivitiesActivitiesPage 112, Q. 2-3, 5Page 112, Q. 2-3, 5 Testing your knowledgeTesting your knowledge
13. 13. Thin LensThin LensEquationsEquations 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
14. 14. Conventions for theConventions for theEquationEquation Distances are measured from the vertex.Distances are measured from the vertex. Focal lengths are positive for converging lensesFocal lengths are positive for converging lensesand negative for diverging lensesand negative for diverging lenses Radii of curvature are positive for convergingRadii of curvature are positive for converginglenses and negative for diverging lenses.lenses and negative for diverging lenses. 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.
15. 15. ActivityActivity Page 122, Q. 5,6,7,9Page 122, Q. 5,6,7,9
16. 16. Devices that UseDevices that UseLensesLenses Many devices rely on lenses, but what isMany devices rely on lenses, but what isa lens?a lens? Circular in shapeCircular in shape Made of transparent materialMade of transparent material When light rays pass through it, they areWhen light rays pass through it, they arerefracted (change direction)refracted (change direction)