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# Physics 504 chapter 3&4 refraction & lenses

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### Physics 504 chapter 3&4 refraction & lenses

1. 1. 11 Chapter 3 & 4Chapter 3 & 4 Refraction &Refraction & LensessLensess Refraction of LightRefraction of Light Chapter 3Chapter 3
2. 2. RefractionRefraction  Refraction is the bending of light as itRefraction is the bending of light as it passes from one medium to anotherpasses from one medium to another medium.medium.  It bends because the light slows downIt bends because the light slows down as it enters theas it enters the new medium.new medium.
3. 3. Table of ObservationsTable of Observations # Activity 1 Invisible coin 2 Broken pencil 3 Glass block 4 From air to water 5 From water to air 6 Dispersion of light 7 Light ray thru’ glass Observations 1.1. Coin disappears whenCoin disappears when viewed from sideviewed from side 2.2. Pencil appears to bendPencil appears to bend 3.3. Coin disappears whenCoin disappears when viewed from sideviewed from side 4.4. Light bends as it entersLight bends as it enters containercontainer 5.5. Light bounces off waterLight bounces off water 6.6. Light spreads out intoLight spreads out into colours ROYGBVcolours ROYGBV 7.7. Light bends twice as itLight bends twice as it enters and exits the glassenters and exits the glass
4. 4. Why is the Sky Red atWhy is the Sky Red at Sunset?Sunset? Dust particles in the air scatter light from the sunDust particles in the air scatter light from the sun and the sky.and the sky.  Blue light is scattered more than red.Blue light is scattered more than red.  When the sun is high in the sky, it looks yellow-When the sun is high in the sky, it looks yellow- white because the light travels vertically through awhite because the light travels vertically through a fairly clear and thin atmosphere, so there is littlefairly clear and thin atmosphere, so there is little refraction.refraction.  At dusk, the sun’s rays travel horizontal andAt dusk, the sun’s rays travel horizontal and through more dust filled air.through more dust filled air.  More of the blue light is scattered so that only theMore of the blue light is scattered so that only the reddish rays hit our eyes.reddish rays hit our eyes.
5. 5. DispersionDispersion  Dispersion is the spreading out of lightDispersion is the spreading out of light into its component colours ROYGBV.into its component colours ROYGBV.  It does this because the different colourIt does this because the different colour light rays slow down at different speedslight rays slow down at different speeds as they enter a new medium.as they enter a new medium.
6. 6. RecompositionRecomposition  Colours of the spectrum may beColours of the spectrum may be recombined by means of a lens to formrecombined by means of a lens to form white light in a process calledwhite light in a process called recomposition.recomposition.  A series of mirrors can shine the differentA series of mirrors can shine the different coloured lights together.coloured lights together.  Newton’s colour disk works because of theNewton’s colour disk works because of the persistence of vision, in which the imagepersistence of vision, in which the image of colours remain on the retina andof colours remain on the retina and combine to form white.combine to form white.
7. 7. RainbowsRainbows  A rainbow is the sun’s spectrumA rainbow is the sun’s spectrum produced by water droplets in theproduced by water droplets in the atmosphere.atmosphere.  The rainbow arc appears atThe rainbow arc appears at specific points in the sky becausespecific points in the sky because only droplets of water that areonly droplets of water that are located along that arc will reflectlocated along that arc will reflect the spectrum at the correct angle inthe spectrum at the correct angle in to the eye of the observer.to the eye of the observer.  The sunlight is behind theThe sunlight is behind the observer, and the viewer mustobserver, and the viewer must angle his/her vision up by 42angle his/her vision up by 42 oo ..  Light enters the spherical rain dropLight enters the spherical rain drop where it is refracted, reflectedwhere it is refracted, reflected internally and finally dispersed.internally and finally dispersed.
8. 8. RefractionRefraction  Refraction is the bending of a light ray asRefraction is the bending of a light ray as it enters a new medium and changesit enters a new medium and changes speed.speed.  The angle is measured from the normal –The angle is measured from the normal – an imaginary line at 90° to the point ofan imaginary line at 90° to the point of intersection.intersection.  Light bends towards the normal if itLight bends towards the normal if it enters an optically denser substance andenters an optically denser substance and v.v.v.v. 88
9. 9. Refractive Index - nRefractive Index - n  The refractive index, n, is a measure of howThe refractive index, n, is a measure of how much light bends as it enters the substance.much light bends as it enters the substance.  n = c/v, where c = 3x10n = c/v, where c = 3x1088 m/sm/s  v = velocity of light in new mediumv = velocity of light in new medium  Air has a refractive index of 1.Air has a refractive index of 1.  Diamond bends light the most (n= 2.42).Diamond bends light the most (n= 2.42).  Table of n values – page 79.Table of n values – page 79. 99
10. 10. Snell’s LawSnell’s Law  In air, n = sin ΘIn air, n = sin Θii / sin Θ/ sin Θrr  Snell’s Law: nSnell’s Law: n11sinΘsinΘ11 = n= n22sinΘsinΘ22  The left side refers to the medium in which theThe left side refers to the medium in which the light is incident.light is incident.  The right side refers to the medium to where theThe right side refers to the medium to where the light exits.light exits.  ActivityActivity  P. 81, Q. 1-3P. 81, Q. 1-3  P. 83, Q. 1-2P. 83, Q. 1-2  P. 86, Q. 3-5, 7P. 86, Q. 3-5, 7 1010
11. 11. Total InternalTotal Internal ReflectionReflection  This occurs when a ray of light passesThis occurs when a ray of light passes from an optically dense material (big n) tofrom an optically dense material (big n) to an optically LESS dense material (low n).an optically LESS dense material (low n).  If the angle of incidence is greater than aIf the angle of incidence is greater than a certain angle – the critical angle - thecertain angle – the critical angle - the light will NOT refract out, but will reflectlight will NOT refract out, but will reflect internally.internally. 1111
12. 12. TOTAL INTERNALTOTAL INTERNAL REFLECTION (TIR)REFLECTION (TIR)
13. 13. Critical AngleCritical Angle  In TIR situations, there comes a point at which theIn TIR situations, there comes a point at which the angle of refraction increases until it leaves theangle of refraction increases until it leaves the medium.medium.  In this case the angle of refraction can be consideredIn this case the angle of refraction can be considered to be equal to 90to be equal to 90oo ..  The angle of incidence at which an angle of refractionThe angle of incidence at which an angle of refraction of 90° first occurs is the Critical Angle.of 90° first occurs is the Critical Angle.  Thus for Critical Angle questions, the angle ofThus for Critical Angle questions, the angle of refraction is assumed to be 90°.refraction is assumed to be 90°. 1313
14. 14. Total InternalTotal Internal ReflectionReflection  1414
15. 15. Snell’s Law & TIRSnell’s Law & TIR  nn11sinΘsinΘ11 = n= n22sinΘsinΘ22  Thus the ΘThus the Θ22 is 90is 90oo , always., always.  The ΘThe Θ11 is called Θis called Θcc , the critical angle., the critical angle.  As n increases, the ΘAs n increases, the Θcc decreases causing moredecreases causing more TIR, which is why diamonds appear so brilliant.TIR, which is why diamonds appear so brilliant.  Page 88, Q. 1, 2, 6Page 88, Q. 1, 2, 6 1515
16. 16. Fibre OpticsFibre Optics  This is especially useful in fibre optics.This is especially useful in fibre optics.  Light enters a optic fibre and reflects onLight enters a optic fibre and reflects on the inside of the cable instead ofthe inside of the cable instead of escaping.escaping.  So signals can be transmitted at theSo signals can be transmitted at the speed of light, much faster than thespeed of light, much faster than the speed of electricity.speed of electricity. 1616
17. 17. Lab ActivityLab Activity  Change: Draw a 10 cm by 10 cm cross (with accurateChange: Draw a 10 cm by 10 cm cross (with accurate 9090oo angles) on a blank sheet of paper.angles) on a blank sheet of paper.  Make a table as in the book up to 60Make a table as in the book up to 60oo ..  In pairs, do the lab.In pairs, do the lab.  Report due beginning of next classReport due beginning of next class   Determine the Critical Angle of Lucite.Determine the Critical Angle of Lucite. 1717
18. 18. Chapter SummaryChapter Summary  Dispersion is the separation of white light into itsDispersion is the separation of white light into its component colours by a prism.component colours by a prism.  The colours of the spectrum, when recombined,The colours of the spectrum, when recombined, form white light.form white light.  White objects tend to reflect light. Black objectsWhite objects tend to reflect light. Black objects tend to absorb light.tend to absorb light.  Subtractive Theory of Colour: the colours of theSubtractive Theory of Colour: the colours of the spectrum contained in white light are subtractedspectrum contained in white light are subtracted by filters or dyes until the desired colourby filters or dyes until the desired colour remains.remains. 1818
19. 19.  Additive Theory of Colour: any otherAdditive Theory of Colour: any other colour of the spectrum can be produced bycolour of the spectrum can be produced by mixing additive primary colours (RBG).mixing additive primary colours (RBG).  IR and UV light are radiations beyond theIR and UV light are radiations beyond the red and violet areas of the spectrum,red and violet areas of the spectrum, respectively. Neither is visible to therespectively. Neither is visible to the human eye.human eye.  Rainbows are formed by the dispersionRainbows are formed by the dispersion and internal reflection of the white lightand internal reflection of the white light from the sun by water droplets in thefrom the sun by water droplets in the atmosphere.atmosphere. 1919
20. 20.  Light refraction terms: incident ray, angleLight refraction terms: incident ray, angle of incidence, normal, reflected ray, angleof incidence, normal, reflected ray, angle of refraction.of refraction.  When light enters a more optically denseWhen light enters a more optically dense medium, its speed decreases.medium, its speed decreases.  n = c/vn = c/v  Law of RefractionLaw of Refraction  Snell’s Law n = sin i / sin rSnell’s Law n = sin i / sin r  The incident ray and refracted ray are onThe incident ray and refracted ray are on opposite sides of the normalopposite sides of the normal 2020
21. 21.  When a ray of light passes into anWhen a ray of light passes into an optically denser medium, it bendsoptically denser medium, it bends towards the normal.towards the normal.  When a ray of light passes into anWhen a ray of light passes into an optically less dense material, it bendsoptically less dense material, it bends away from the normal.away from the normal.  When a ray of light passes into aWhen a ray of light passes into a medium at an angle of 90medium at an angle of 90۫۫, it does not, it does not refract.refract. 2121
22. 22.  Total Internal Reflection TIRTotal Internal Reflection TIR  The ray of light passes from one mediumThe ray of light passes from one medium into another of lower optical density.into another of lower optical density.  The angle of incidence is greater than theThe angle of incidence is greater than the critical angle.critical angle.  When the angle of incidence is greater thanWhen the angle of incidence is greater than the critical angle, the refracted angle is 90the critical angle, the refracted angle is 90۫۫..  nn11sin Θsin Θ11 = n= n22 sin Θsin Θ22  Answer:Answer: P. 93, Q. 1-7P. 93, Q. 1-7  2222
23. 23. Types of LensesTypes of Lenses  Converging:Converging:  Causes light rays to come togetherCauses light rays to come together  Examples: Magnifying glass, camera, eyeExamples: Magnifying glass, camera, eye glassesglasses  Diverging:Diverging:  Causes light rays to spread apartCauses light rays to spread apart  Examples: Microscope, telescope, eyeExamples: Microscope, telescope, eye glassesglasses
24. 24. Principal RaysPrincipal Rays  Just like curved mirrors, lenses are used toJust like curved mirrors, lenses are used to produce imagesproduce images  The images are found by using three principalThe images are found by using three principal rays:rays:  Principal Ray 1:Principal Ray 1: A light ray parallel to the principalA light ray parallel to the principal axis 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 other side.side.  Principal Ray 3:Principal Ray 3: Passes through the secondaryPasses through the secondary focal point and will be refracted. Will exit the lensfocal point and will be refracted. Will exit the lens parallel to the principal axis.parallel to the principal axis.
25. 25. Converging LensConverging Lens
26. 26. LensLens 
27. 27. 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 the prolongation of the ray will pass through theprolongation of the ray will pass through the principal focus.principal focus.  Principal ray 2Principal ray 2 (straight through the optical(straight through the optical center) 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.
28. 28. Diverging LensDiverging Lens  Answer: Page 98, Q. 1-3
29. 29. Optical PowerOptical Power  The strength of a lens (optical power) is relatedThe strength of a lens (optical power) is related to its focal length. A short focal length meansto its focal length. A short focal length means the light rays are being refracted a lot. Thethe light rays are being refracted a lot. The optical 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 not being refracted very much. The optical powerbeing refracted very much. The optical power of 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 of the 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
30. 30. Lens CombinationsLens Combinations  By combining lenses, different optical powers can beBy combining lenses, different optical powers can be obtained. 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 uses more 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 to solve optical power. You must use the equationsolve optical power. You must use the equation P=1/fP=1/f to obtain theto obtain the PP value.value.
31. 31. Lens CombinationsLens Combinations  What is the power, focal lengths?What is the power, focal lengths?
32. 32. ActivitiesActivities Page 112, Q. 2-3, 5Page 112, Q. 2-3, 5  Testing your knowledgeTesting your knowledge
33. 33. Thin LensThin Lens EquationsEquations  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
34. 34. Conventions for theConventions for the EquationEquation  Distances are measured from the vertex.Distances are measured from the vertex.  Focal lengths are positive for converging lenses andFocal lengths are positive for converging lenses and negative for diverging lensesnegative for diverging lenses  Radii of curvature are positive for converging lenses andRadii of curvature are positive for converging lenses and negative for diverging lenses.negative for diverging lenses.  Image and object distances are positive for real imagesImage and object distances are positive for real images and objects.and objects.  Image and object distances are negative for virtualImage and object distances are negative for virtual images and objects.images and objects.  Image and object heights are positive when upright andImage and object heights are positive when upright and negative when inverted.negative when inverted.  Answer: Page 122, Q. 5,6,7,9Answer: Page 122, Q. 5,6,7,9
35. 35. Devices that UseDevices that Use LensesLenses  Many devices rely on lenses, but what isMany devices rely on lenses, but what is a 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 are refracted (change direction)refracted (change direction)