Light and sight


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Light and sight

  1. 1. Focus questionsWhat is the wave nature of light?How does light behave?What are the primary colors of light? of pigments?How are images formed?
  2. 2. Light and Sight
  3. 3. Light And SightProperties of lightBehavior of light Reflection Formation of images in mirrors Refraction Color Formation of images in lensesThe Human Eye
  4. 4. Light a form of energy
  5. 5. Part 1 – Properties of Light Light travels in straight lines: Laser
  6. 6. Light travels VERY FAST – around 300,000 kilometres per second.At this speed it cango around the world 8times in one second.
  7. 7. Light travels much faster than sound. For example: 1) Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.
  8. 8. We see things because they reflect light into our eyes: Homework
  9. 9. Luminous and non-luminous objects A luminous object is one that produces light.A non-luminous object is one that reflects light. Luminous objects Reflectors
  10. 10. ShadowsShadows are places where light is “blocked”: Rays of light
  11. 11. Properties of Light summary1) Light travels in straight lines2) Light travels much faster than sound3) We see things because they reflect light into our eyes4) Shadows are formed when light is blocked by an object
  12. 12. Behavior of Light:Reflection
  13. 13. Behavior of Light Reflection from a mirror: NormalIncident ray Reflected ray Angle of Angle of incidence reflection Mirror
  14. 14. The Law of Reflection Angle of incidence = Angle of reflectionIn other words, light gets reflected from a surface at____ _____ angle it hits it. The same !!!
  15. 15. Clear vs. Diffuse ReflectionSmooth, shiny surfaces have a clear reflection:Rough, dull surfaces havea diffuse reflection.Diffuse reflection is whenlight is scattered indifferent directions
  16. 16. Behavior of LightImage formation in mirrors
  17. 17. The Law of Reflection For specular reflection the incident angle θi equals the reflected angle θr: θi = θrThe angles are measured relative to the normal, shown here as a dotted line.
  18. 18. Image formed by a Plane Mirror A mirror is an object that reflects light. A plane mirror is simply a flat mirror. Consider an object placed at point P in front of a plane mirror. An image will be formed at point P´ behind the = distance from object to mirrordi = distance from image to mirrorho = height of objecthi = height of imageFor a plane mirror: hi do = -di and ho = hi Image is behind mirror: di < 0
  19. 19. ImagesAn image is formed at the point where the rays of light leaving a single point on an object either actually intersect or where they appear to originate from.If the light rays actually do intersect, then the image is a real image. If the light only appears to be coming from a point, but is not physically there, then the image is a virtual image.We define the magnification, m, of an image to be: image height hi di m= = =− object height ho doIf m is negative, the image is inverted (upside down).
  20. 20. Plane MirrorsA plane mirror image has the following properties:The image distance equals the object distance ( in magnitude )The image is unmagnified image height hi d m= = =− i object height ho doThe image is virtual: do di  negative image distance  di < 0m>0, The image is not inverted
  21. 21. Curved or Spherical mirrors
  22. 22. Curved MirrorsA curved mirror is a mirror whose surface shape is spherical with radius of curvature R. There are two types of spherical mirrors: concave and convex.We will always orient the mirrors so that the reflecting surface is on the left. The object will be on the left.
  23. 23. Focal PointWhen parallel rays (e.g. rays from a distance source) are incident upon a spherical mirror, the reflected rays intersect at the focal point F, a distance R/2 from the mirror.For a concave mirror, the focal point is in front of the mirror (real).For a convex mirror, the focal point is behind the mirror The incident rays diverge (virtual). from the convex mirror, but they trace back to a virtual focal point F.
  24. 24. Focal LengthThe focal length f is the distance from the surface of the mirror to the focal point.CF = FA = radius = FMThe focal length FM is half the radius of curvature of a spherical mirror.Sign Convention: the focal length is negative if the focal point is behind the mirror.For a concave mirror, f = ½RFor a convex mirror, f = −½R (R is always positive)
  25. 25. RayTracingIt is sufficient touse two of fourprincipal rays todetermine wherean image will belocated. The parallel ray (P ray) reflects through the focal point. The focal ray (F ray) reflects parallel to the axis, and the center-of- curvature ray (C ray) reflects back along its incoming path. The Mid ray (M ray) reflects with equal angles at the axis of symmetry of the mirror.
  26. 26. Ray Tracing – ExamplesReal image Virtual image
  27. 27. The Mirror EquationThe ray tracing technique Sign Conventions: shows qualitatively where the image will be located. do is positive if the object is in The distance from the front of the mirror (real object) mirror to the image, di, do is negative if the object is in can be found from the back of the mirror (virtual mirror equation: object) 1 1 1 di is positive if the image is in front + = do di f of the mirror (real image)do = distance from object to di is negative if the image is behind mirror the mirror (virtual image)di = distance from image to f is positive for concave mirrors mirror f is negative for convex mirrors dif = focal length m = − d m is positive for upright images o
  28. 28. Using mirrorsTwo examples: 2) A car headlight 1) A periscope
  29. 29. How do you explain these?
  30. 30. RefractionIncident ray Refracted ray Emergent ray
  31. 31. Refraction
  32. 32. Refraction
  33. 33. The Refraction of LightThe speed of light is different in differentmaterials. We define the index of refraction, n,of a material to be the ratio of the speed oflight in vacuum to the speed of light in thematerial: n = c/vWhen light travels from one medium to anotherits velocity and wavelength change, but itsfrequency remains constant.
  34. 34. Snell’s Law In general, when light enters a new material its direction will change. The angle of refraction θ2 is related to the angle of incidence θ1 by Snell’s Law: sin θ1 sin θ2 = = constant v1 v2 where v is the velocity of light in the medium. Snell’s Law can also be written as n1sinθ1 = n2sinθ2The angles θ1 and θ2 are measured relative to the line normal to the surface between the two materials.
  35. 35. ColourWhite light is not a single colour; it is made up of a mixture of the seven colours of the rainbow.We can demonstrate this bysplitting white light with aprism:This is how rainbows areformed: sunlight is “split up”by raindrops.
  36. 36. The colours of the rainbow: Red Orange Yellow Green Blue Indigo Violet
  37. 37. Why are objects colored?
  38. 38. Adding colours White light can be split up to make separate colours. These colours can be added together again. The primary colours of light are red, blue and green:Adding blue and red Adding blue andmakes magenta green makes cyan(purple) (light blue)Adding red Adding alland green three makesmakes yellow white again
  39. 39. Seeing colourThe colour an object appears depends on the colours of light it reflects.For example, a red book only reflects red light: White Only red light light is reflected
  40. 40. A pair of purple trousers would reflect purple light(and red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colours: White light
  41. 41. Using coloured lightIf we look at a coloured object in coloured light we see something different. For example, consider a this pair of shirt and shorts: Shirt looks red White light Shorts look blue
  42. 42. In different colours of light they would look different: Red Shirt looks red light Shorts look black Shirt looks black Blue light Shorts look blue
  43. 43. Some further examples: Colour objectObject Colour of light seems to be Red RedRed socks Blue Black Green Black Red BlackBlue teddy Blue Green RedGreen camel Blue Green RedMagenta book Blue Green
  44. 44. Using filtersFilters can be used to “block” out different colours of light: Red Filter Magenta Filter
  45. 45. Investigating filtersColour of filter Colours that could be “seen” Red Green Blue Cyan Magenta Yellow
  46. 46. Red Blue Green WhiteYellow Cyan Magenta
  47. 47. LensesLight is reflected from a mirror. Light is refracted through a lens.
  48. 48. Focal PointThe focal point of a lens is the point where parallel rays incident upon the lens converge. converging lens diverging lens
  49. 49. How does a lens form an image?
  50. 50. Ray Tracing for Lenses Just as for mirrors we use three “easy” rays to find the image from a lens. The lens is assumed to be thin.The P ray propagates parallel to the principal axis until itencounters the lens, where it is refracted to pass through thefocal point on the far side of the lens. The F ray passes throughthe focal point on the near side of the lens, then leaves the lensparallel to the principal axis. The M ray passes through themiddle of the lens with no deflection (in thin lens limit).
  51. 51. The Lens Equation 
  52. 52. Data and Analysis How does the image distance q of a convex lens change as the object distance p is decreased?How does the height of the image change as the object distance is decreased?Write the equation in determining the linear magnification m of a convex lens using p and q. Call this Eq (1) 
  53. 53. Data and Analysis Using Excel graph m vs q. What does the graph show?. When m is zero, what is q?. This is the q intercept. Relate this observation with your answer to (c). Compare the value of the q intercept with the focal length of the mirror. What then is the physical meaning of the q- intercept?Extend your graph until it intersects the vertical axis m. Compute the slope of your graph. Compare the value of the slope and the q-intercept. How are they related?. Express the relationship IN AN EQAUTION using the physical meaning of the q – intercept you found. Call this Eq (2).Write the equation of the line graph. Call this Eq (3)
  54. 54. Make a data tablep q M = f p – object distance q/p2 2 1 1 q – image distance1.5 3 2 f – focal length1.23 5.16 4.2 m - magnification1.8 2.25 1.252.2 1.83 .832.4 1.71 .712.7 1..58 .592.8 1.55 .55
  55. 55. q q/p = m2 13 25.16 4.22.25 1.251.83 .831.71 .711.58 .591.55 .55
  56. 56. The Thin Lens EquationThe ray tracing technique shows 1 1 1qualitatively where the image from a lenswill be located. The distance from the 1+ 1 = 1 do + di = flens to the image, can be found from the do di fthin-lens equation:Sign Conventions:do is positive for real objects (from which light diverges)do is negative for virtual objects (toward which light converges)di is positive for real images (on the opposite side of the lens from the object)di is negative for virtual images (same side as object)f is positive for converging (convex) lensesf is negative for diverging (concave) lensesm is positive for upright images
  57. 57. The eye and the camera
  58. 58. The eye model
  59. 59. The Human EyeThe human eye is a dynamic optical device that adjusts its focal length to keep the image location positioned at the retina:
  60. 60. Optics of the Eye1. The “normal” eye can be modeled as a simple lens system with an effective focal length (& optical power) and a fixed image distance, i: 1 1 1 = + f p 0.018m2. The job of the eye is to focus images on the retina. The image distance is therefore fixed at 1.8 cm (or 0.018 m).3. When the eye cannot adequately focus an image on the retina, correction may be needed4. The 4 common vision problems: a. Myopia (near sightedness, short far & near point) b. Hypermetropia (far sightedness, long far & near point) c. Astigmatism (warped lens optics, focal length not uniform on all axes in the eye) d. Presbyopia (normal distance vision but inability to accommodate for close objects)
  61. 61. Distance Vision Optics1. When viewing distant objects, the lens power of the eye (& focal length) of the eye is given by: 1 1 1 1 = + = = 55.6 m-1 f ∞ 0.018 m 0.018 m2. The lens power is 55.6 diopters & the focal length is: f = 0.018 m1. When a person is near sighted (myopic), he/she cannot see objects at infinity (“infinity” is the “far point” for a normal eye) – Myopic far point < Normal far point
  62. 62. Distance Vision OpticsExample: A person with -2.0 diopter distance correction.a. This person has a lens power of 57.6 & needs this “minus” correction to lower the effective lens power to a “normal” 55.6: 1 1 1 = + = 57.6 diopters ⇒ p = 2.0 m f p 0.018 mb. The far point for this person is: p = 2 m {any object beyond this distance is not in focus}
  63. 63. Near Vision Optics1. When viewing close-in objects, the lens power of the eye (& focal length) of the eye is given by: 1 1 1 = + = 59.6 m-1 f 0.25 m 0.018 m2. The lens power is 59.6 diopters & the focal length is: f = 0.0168 m3. A far sighted (hyperopic) person cannot see objects at close distances even though the eye is accomodating normally – Hyperopic near point > Normal near point (0.25 m)
  64. 64. Near Vision OpticsExample: A person with +2.0 diopter vision correction.a. This person has a (near) lens power of 57.6 & needs this “plus” correction to raise the effective lens power to a “normal” close distance power of 59.6: 1 1 1 = + = 57.6 diopters ⇒ p = 0.49 m f p 0.018 mb. The near point for this person is: p = 0.49 m {any object closer is not in focus}c. People w/presbyopia have normal distance lens power but are unable to adjust for closer objects, thus needing “reader” glasses