Optics basics concepts

it is a postgraduate base lecture

1. OPTICS: BASICS CONCEPTS Md Anisur Rahman (Anjum) Professor & Head of the department (Ophthalmology) Dhaka Medical College, Dhaka

2. What is optical science. Optical science . Though most people associate the word ‘optics’ with the engineering of lenses for eyeglasses, telescopes, and microscopes, In physics the term more broadly refers to   the study of the behavior of light and its interactions with matter .

3. Three broad subfields of optics Geometrical optics , the study of light as rays Physical optics , the study of light as waves Quantum optics , the study of light as particles

4. Geometrical optics Light is postulated to travel along  rays  – line segments which are straight in free space but may change direction, or even curve, when encountering matter.

5. Geometrical optics Two laws dictate what happens when light encounters a material surface. The  law of reflection , evidently first stated by Euclid around 300 BC, states that when light encounters a flat reflecting surface the angle of incidence of a ray is equal to the angle of reflection.

6. 1. Geometrical optics The law of refraction , experimentally determined by Willebrord Snell in 1621, explains the manner in which a light ray changes direction when it passes across a planar boundary from one material to another.

7. From the laws of reflection and refraction: One can determine the behavior of optical devices such as telescopes and microscopes. One can trace the paths of different rays (known as ‘ray tracing’) through the optical system

8. How images can be formed? Their relative orientation, and their magnification. This is in fact the most important use of geometrical optics to this day: the behavior of complicated optical systems can, to a first approximation, be determined by studying the paths of all rays through the system.

9. 2. Physical optics Looking again at the ray picture of focusing above, we run into a problem: at the focal point, the rays all intersect. The density of rays at this point is therefore infinite, which according to geometrical optics implies an  infinitely bright focal spot . Obviously, this cannot be true.

10. If we put a black screen in the plane of the focal point and look closely at the structure of the focal spot projected on the plane, experimentally we would see an image as simulated below:

11. There is a very small central bright spot, but also much fainter (augmented in this image) rings surrounding the central spot. These rings cannot be explained by the use of geometrical optics alone, and result from the wave nature of light.

12. Physical optics is the study of the wave properties of light, which may be roughly grouped into three categories: Interference, Diffraction, and Polarization. 

13.   Interference   Interference  is the ability of a wave to interfere with itself, creating localized regions where the field is alternately extremely bright and extremely dark.

14. Diffraction Diffraction  is the ability of waves to ‘bend’ around corners and spread after passing through an aperture. 

15. Polarization   Polarization  refers to properties of light related to its transverse nature. We will cover all these terms in more detail in subsequent posts.

16. Quantum optics We return to the picture of the focal spot illustrated above and now imagine that the light source which produces the focal spot is on a very precise dimmer switch. What happens as we slowly turn the dimmer switch down to the off position?

17. Physical optics predicts that the shape of the focal spot will remain unchanged; it will just grow less bright. When the dimmer switch is turned below some critical threshold, however, something different and rather unexpected happens: we detect light in little localized ‘squirts’ of energy, and do not see our ring pattern at all.

18. If we keep a running tally of how many squirts hit at each location, we can slowly build up an average picture of where light energy is being deposited in above figure.

19. Geometric Optics Geometric Optics deals with the formation of images by using such optical devices as lenses, prisms and mirrors and with the laws governing the characteristics of these images, such as their size, shape, position and clarity. Rays of light Pencil of light Beam of light (M.A MATIN P=19) 17 March 2017 21 [email protected]

20. Reflection The  law of reflection , evidently first stated by Euclid around 300 BC, states that when light encounters a flat reflecting surface the angle of incidence of a ray is equal to the angle of reflection

21. Reflection of light When light meets an interface between two media, its behavior depends on the nature of the two media involved. Light may be absorbed by the new medium or transmitted onward through it or it may bounch back into first medium. This bouncing of light at an interface is called Reflection. (M.A MATIN = 21) 17 March 2017 23 [email protected]

22. Q. What happened to the light when it strikes a surface? Ans) 3 things may happen. It may be: Absorbed Reflected Or Refracted

23. Defination of Reflection Reflection is defined as the change of path of light without any change in the medium. All the reflections end up in producing images of the object kept in front of the reflecting surface.

24. Laws of Reflection The incidence ray and the reflected ray lie in the same plane which is perpendicular to the mirror surface at the point of incidence. When light is reflected off any surface, the angle of incidence is always equal to the angle of reflection, 17 March 2017 26 [email protected]

25. Mirror A mirror is optical media which reflects light backwards when fall on it. It may be: Plane mirrors or Spherical mirrors. 17 March 2017 27 [email protected]

26. Mirror: Rules for rays tracing through a mirror The ray which pass through the pole shall pass undeviated. The ray which is parallel with the axis shall pass through the focal point after convergence or divergence. The ray passing through the focal point & falling on the mirror surface shall pass parallel to the optical axis. The ray passing through the centre of curvature of a mirror shall also pass undeviated. Path of light rays are also reversible. 17 March 2017 28 [email protected]

27. Reflection at a plane surface 17 March 2017 29 [email protected]

28. Spherical Mirrors Silvering a piece of glass which would form part of the shell of a hollow sphere. Silvering the glass on the outside gives a concave or converging mirror, while silvering on the inside gives a convex or diverging mirror. 17 March 2017 30 [email protected]

29. Types of images There are two types of images formed mirrors. They are: 1) Virtual image. 2) Real image.

30. Virtual image Virtual image can not be focused on a screen. It is always upright. No light is really passing through the apparent location of the image. The virtual image formed by plane mirror is laterally inverted

31. Real image Real image can be focus on a screen. It is always inverted. The light passes through the location of the image.

32. Nomenclature Light rays falling on the surface are called incident rays. Light rays travelling back are called reflected rays. A line at right angle to the reflecting surface is called normal Light travelling along the normal is reflected back along the normal

33. angle of incident . angle of reflection .

34. Nomenclature 5) The angle formed by the incident ray and the normal is called angle of incident. 6) The angle formed by the reflected ray and the normal is called angle of reflection. 7) The angle of incident and the angle of reflection are equal.

35. Nomenclature 8) The incident ray, the reflected ray and the normal are in the same plane. 9) The line joining the centre of curvature to any point on the curved mirror is the normal of that mirror. 10) The focal length of the plane mirror is infinity.

36. Image formation by plain mirror If the reflecting surface of the mirror is flat then we call this type of mirror as  plane mirrors . Light always has regular reflection on plane mirrors. Given picture below shows how we can find the image of a point in plane mirrors.

37. Characteristics of image formed by a plane mirror. Image is virtual and erect. It is of same size as the object. It has the same distance as object to the mirror. It is laterally reversed. The minimum length of the mirror required to form full size image of the object is half the size of the object.

38. Number of images How many images can you form by two plane mirror? It depends upon the inclination of two mirrors with each other. The number of images formed by two plane mirrors inclined to each other is calculated by the formula:

39. Number of images N=360/ ᴓ - 1 (Here, N = number of images form, ᴓ is the angle between two mirrors) Less the angle between two mirrors, more the number of images.

40. Number of images N = 360/90 – 1 = 4 – 1 = 3. N = 360/60 – 1 = 6 – 1 = 5 N= 360/45 – 1 = 8 – 1 = 7. An object placed between two parallel plane mirrors will form infinite number of images. This is true only for mirrors kept at right angles or less than that.

41. Uses of plane mirror in ophthalmology A plane mirror is used at a distance of 3 m with a reverse Snellen’s chart kept at little higher position than patient’s head. Used in plane mirror retinoscope. Used in both direct & indirect ophthalmoscope. Used in slit lamp, synaptophore, stereoscope, to change the direction of rays & save space.

42. Spherical mirror Pole Center of curvature

43. Nomenclature in spherical mirror image Pole: It is the vertex of the mirror. Center of curvature: It is the center of curvature of the sphere out of which the mirror is fashioned. Radius of curvature: It is the line joining the center of curvature to the pole. Principal axis: It is the ling joining center of curvature and the vertex.

44. Nomenclature in spherical mirror image 5) Normal in a spherical mirror: It is a line that joins any point of the mirror to the center of curvature. 6) All the measurements are valid from the pole of the center. 7) By convention, all the incident rays are taken to travel from the left to right.

45. Nomenclature in spherical mirror image 8) Focal length of a concave mirror is taken as negative and positive in convex lens

46. The principal axis of a spherical mirror is the line joining the pole P or centre of the mirror to the centre of curvature C which is the centre of the sphere of which the mirror forms a part. P C 17 March 2017 50 [email protected]

47. radius of curvature r The radius of curvature r is the distance CP. In the case of a concave mirror the centre of curvature is in front of the mirror ; in a convex mirror it is behind. 17 March 2017 51 [email protected]

48. Principal Focus When a parallel beam of light falls on a plane mirror it is reflected as a parallel beam ; but in the case of a concave mirror the rays in a parallel beam are all reflected so as to converge to a point called a focus. If the incident rays are parallel to the principal axis the point through which all the reflected rays pass is on the principal axis just midway between the pole and the centre of curvature and is called the principal focus F. 17 March 2017 52 [email protected]

49. What happens when a beam of light parallel to the principal axis falls on a convex mirror? In this case the rays are reflected so that they all appear to be coming from a principal focus midway between the pole and centre of curvature behind the mirror. 17 March 2017 53 [email protected]

50. A concave mirror, therefore has a real principal focus, while the convex mirror has a virtual one. The focal length of a spherical mirror is half its radius of curvature. 17 March 2017 54 [email protected]

51. Construction of ray diagrams Since a point on an image can be located by the point of intersection of two reflected rays, we have to consider which are the most convenient rays to use for this purpose. Remembering that, by geometry, the normal to a curved surface at any point is the radius of curvature at that point, one very useful ray to draw will be one which is incident along a radius of curvature. Since this is incident normally on the mirror, it will be reflected back along its own path. 17 March 2017 55 [email protected]

52. Construction of ray diagrams Another useful ray is one which falls on the mirror parallel to the principal axis. By definition, this will be reflected through the principal focus. Conversely, any incident ray passing through the principal focus will be reflected back parallel to the principal axis. The same observations also apply to the convex mirrors, so we may briefly sum them up into a set of rules for constructing images formed by spherical mirrors. 17 March 2017 56 [email protected]

53. Construction of ray diagrams Rays passing through the centre of curvature are reflected back along their own paths. Rays parallel to the principal axis are reflected through the principal focus. Rays through the principal focus are reflected parallel to the principal axis. (Useful when using squared paper) Rays incident at the pole are reflected, making the same angle with the principal axis. 17 March 2017 57 [email protected]

54. Images formed by a concave mirror . We wish to describe the characteristics of the image for any given object location. The L of L•O•S•T represents the relative location. The O of L•O•S•T represents the orientation (either upright or inverted) . The S of L•O•S•T represents the relative size (either magnified, reduced or the same size as the object) . And the T of L•O•S•T represents the type of image (either real or virtual) . The best means of summarizing this relationship between object location and image characteristics is to divide the possible object locations into five general areas or points: 17 March 2017 58 [email protected]

55. Images formed by a concave mirror Case 1: the object is located beyond the center of curvature (C) Case 2: the object is located at the center of curvature (C) Case 3: the object is located between the center of curvature (C) and the focal point (F) Case 4: the object is located at the focal point (F) Case 5: the object is located in front of the focal point (F) 17 March 2017 59 [email protected]

56. 17 March 2017 60 [email protected]

57. Case 1: The object is located beyond C When the object is located at a location beyond the center of curvature, the image will always be located somewhere in between the center of curvature and the focal point. In this case, the image will be an inverted image . reduced in size ; 17 March 2017 61 [email protected]

58. The object is located beyond C (contd) Finally, the image is a real image . Light rays actually converge at the image location. If a sheet of paper were placed at the image location, the actual replica of the object would appear projected upon the sheet of paper. 17 March 2017 [email protected] 62

59. Case 1: The object is located beyond C 17 March 2017 63 [email protected]

60. Case 2: The object is located at C When the object is located at the center of curvature, the image will also be located at the center of curvature. In this case, the image will be inverted. The image dimensions are equal to the object dimensions. Finally, the image is a real image . 17 March 2017 64 [email protected]

61. Case 2: The object is located at C 17 March 2017 65 [email protected] Light rays actually converge at the image location. As such, the image of the object could be projected upon a sheet of paper.

62. Case 3: The object is located between C and F When the object is located in front of the center of curvature, the image will be located beyond the center of curvature . In this case, the image will be inverted. The image dimensions are larger than the object dimensions. 17 March 2017 66 [email protected]

63. Case 3: The object is located between C and F 17 March 2017 67 [email protected] Finally, the image is a real image . Light rays actually converge at the image location. As such, the image of the object could be projected upon a sheet of paper.

64. Case 4: The object is located at F When the object is located at the focal point, no image is formed. Light rays from the same point on the object will reflect off the mirror and neither converge nor diverge. After reflecting, the light rays are traveling parallel to each other and do not result in the formation of an image. 17 March 2017 68 [email protected]

65. Case 4: The object is located at F 17 March 2017 69 [email protected]

66. Case 5: The object is located in front of F When the object is located at a location beyond the focal point, the image will always be located somewhere on the opposite side of the mirror. Regardless of exactly where in front of F the object is located, the image will always be located behind the mirror. In this case, the image will be an upright image, magnified and virtual 17 March 2017 70 [email protected]

67. Case 5: The object is located in front of F 17 March 2017 71 [email protected]

68. Case 5: The object is located in front of F This type of image is formed by a shaving or make-up mirror and also by small concave mirror used by dentists for examining teeth. 17 March 2017 72 [email protected]

69. Case 5: The object is located in front of F Light rays from the same point on the object reflect off the mirror and diverge upon reflection. For this reason, the image location can only be found by extending the reflected rays backwards beyond the mirror. The point of their intersection is the virtual image location. It would appear to any observer as though light from the object were diverging from this location. Any attempt to project such an image upon a sheet of paper would fail since light does not actually pass through the image location. 17 March 2017 73 [email protected]

70. It might be noted from the above descriptions that there is a relationship between the object distance and object size and the image distance and image size. Starting from a large value, as the object distance decreases (i.e., the object is moved closer to the mirror), the image distance increases; meanwhile, the image height increases. 17 March 2017 74 [email protected]

71. At the center of curvature, the object distance equals the image distance and the object height equals the image height. As the object distance approaches one focal length, the image distance and image height approaches infinity. Finally, when the object distance is equal to exactly one focal length, there is no image. 17 March 2017 75 [email protected]

72. Then altering the object distance to values less than one focal length produces images that are upright, virtual and located on the opposite side of the mirror. Finally, if the object distance approaches 0, the image distance approaches 0 and the image height ultimately becomes equal to the object height. 17 March 2017 76 [email protected]

73. Nine different object locations are drawn and labeled with a number; the corresponding image locations are drawn in blue and labeled with the identical number. 17 March 2017 [email protected] 77

74. 17 March 2017 78 [email protected]

75. IMAGE FORM BY CONVEX MIRROR 17 March 2017 79 [email protected]

76. IMAGE FORM BY CONVEX MIRROR 17 March 2017 80 [email protected]

77. IMAGE FORM BY CONVEX MIRROR The diagrams above show that in each case, the image is located behind the convex mirror a virtual image an upright image reduced in size (i.e., smaller than the object) 17 March 2017 81 [email protected]

78. IMAGE FORM BY CONVEX MIRROR Unlike concave mirrors , convex mirrors always produce images that share these characteristics. The location of the object does not affect the characteristics of the image. As such, the characteristics of the images formed by convex mirrors are easily predictable. 17 March 2017 82 [email protected]

79. IMAGE FORM BY CONVEX MIRROR Another characteristic of the images of objects formed by convex mirrors pertains to how a variation in object distance affects the image distance and size. The diagram below shows seven different object locations (drawn and labeled in red) and their corresponding image locations (drawn and labeled in blue). 17 March 2017 83 [email protected]

80. IMAGE FORM BY CONVEX MIRROR 17 March 2017 84 [email protected]

81. IMAGE FORM BY CONVEX MIRROR The diagram shows that as the object distance is decreased, the image distance is decreased and the image size is increased. So as an object approaches the mirror, its virtual image on the opposite side of the mirror approaches the mirror as well; and at the same time, the image is becoming larger. 17 March 2017 85 [email protected]

82. Image formed by concave mirror Position of the object Position of the image Nature of the image Inverted/ Erect Size Between focus & pole Behind the mirror Virtual Erect Magnified At focus Infinity Real Inverted Highly Magnified Between focus & curvature Beyond center of curvature Real Inverted Little Magnified

83. Image formed by concave mirror Position of the object Position of the image Nature of the image Inverted/ Erect Size Center of curvature Same place Real Inverted Same size Beyond the center of curvature Between focus & center of curvature Real Inverted Diminished At infinity Real Inverted Very small

84. Image formed by convex mirror The image of an object kept in front of the mirror is formed behind the mirror. It is smaller than the object , erect and virtual. The distance between the image and the mirror is less than between the object and the mirror.

85. Behavior of images in relation to position of the object The image formed by CONVEX and PLANE mirrors are virtual The image formed by CONCAVE mirrors can be real or virtual The distance between mirror and the image is least in CONVEX mirror, most in CONCAVE mirror and equal in PLANE mirror

86. specular reflection & diffuse reflection Reflection of smooth surfaces such as mirrors or a calm body of water leads to a type of reflection known as  specular reflection . Reflection of rough surfaces such as clothing, paper, and the asphalt roadway leads to a type of reflection known as  diffuse reflection .

87. Whether the surface is microscopically rough or smooth has a tremendous impact upon the subsequent reflection of a beam of light.

88. specular reflection & diffuse reflection The diagram depicts two beams of light incident upon a rough and a smooth surface.

89. Applications of Specular and Diffuse Reflection There are several interesting applications of this distinction between specular and diffuse reflection. One application pertains to the relative difficulty of night driving on a wet asphalt roadway compared to a dry asphalt roadway. Most drivers are aware of the fact that driving at night on a wet roadway results in an annoying glare from oncoming headlights.

90. Applications of Specular and Diffuse Reflection The glare is the result of the specular reflection of the beam of light from an oncoming car. Normally a roadway would cause diffuse reflection due to its rough surface. But if the surface is wet, water can fill in the crevices and smooth out the surface.

91. Applications of Specular and Diffuse Reflection Rays of light from the beam of an oncoming car hit this smooth surface, undergo specular reflection and remain concentrated in a beam. The driver perceives an annoying glare caused by this concentrated beam of reflected light.

92. Applications of Specular and Diffuse Reflection A second application of the distinction between diffuse and specular reflection pertains to the field of photography. Many people have witnessed in person or have seen a photograph of a beautiful nature scene captured by a photographer who set up the shot with a calm body of water in the foreground.

93. Applications of Specular and Diffuse Reflection The water (if calm) provides for the specular reflection of light from the subject of the photograph.

94. Applications of Specular and Diffuse Reflection Light from the subject can reach the camera lens directly or it can take a longer path in which it reflects off the water before traveling to the lens. Since the light reflecting off the water undergoes specular reflection, the incident rays remain concentrated (instead of diffusing).

95. Applications of Specular and Diffuse Reflection The light is thus able to travel together to the lens of the camera and produce an image (an exact replica) of the subject which is strong enough to perceive in the photograph. An example of such a photograph is shown.

96. Question If a bundle of parallel incident rays undergoing diffuse reflection follow the law of reflection, then why do they scatter in many different directions after reflecting off a surface?

97. Answer Each individual ray strikes a surface which has a different orientation. Since the normal is different for each ray of light, the direction of the reflected ray will also be different.

98. Question Perhaps you have observed magazines which have glossy pages. The usual microscopically rough surface of paper has been filled in with a glossy substance to give the pages of the magazine a smooth surface. Do you suppose that it would be easier to read from rough pages or glossy pages? Explain your answer.

99. It is much easier to read from rough pages which provide for diffuse reflection. Glossy pages result in specular reflection and cause a glare. The reader typically sees an image of the light bulb which illuminates the page. If you think about, most magazines which use glossy pages are usually the type which people spend more time viewing pictures than they do reading articles.

100. Refraction

101. Luminous versus Illuminated Objects The objects that we see can be placed into one of two categories: luminous objects and illuminated objects. Luminous objects  are objects that generate their own light Illuminated objects  are objects that are capable of reflecting light to our eyes.

102. The sun is an example of a luminous object, while the moon is an illuminated object.

103. Refraction Q) What happened to the light when it strikes a surface? Ans) 3 things may happen. It may be: Absorbed Reflected Or Refracted

104. Refraction Q) What is refraction? Ans) Refraction of light is a phenomenon of change in the path of light when it passes from one medium to another due to change in velocity.

105. Terms used in refraction NORMAL: This is a line right angles to the interface INCIDENCE RAY: The ray that strikes the interface at the base of the normal in an angular fashion. REFRACTED RAY : This is the deviated ray in the second medium.

106. ANGLE OF INCIDENCE: Angle between the normal and the incident ray ANGLE OF REFRACTION: The angle between the refracted ray & the normal is called ANGLE OF REFRACTION 6) The two angles are never equal.

107. Snell’s Law

108. Total Internal Reflection (TIR)

109. Critical Angle Critical angle is the angle of incidence above which total internal reflection occurs. It is defined as the angle when the incidence ray is of such an angle that the refracted ray is at right angles to the normal

110. Critical Angle Critical angle of glass is 48.6 , diamond is 24 (refractive index is 2.42) and water is 48.75 . An incident ray when passing through a slab of glass with air on either side will exit the slab as refracted ray and will be parallel to incident ray.

111. Total Internal Reflection (TIR) The complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.

112. Total Internal Reflection (TIR)

113. Different uses of TIR Gonioscopy employs total internal reflection to view the anatomical angle formed between the eye's cornea and iris. Total internal reflection is the operating principle of optical fibers, which are used in endoscopes and telecommunications.

114. Different uses of TIR 3) Total internal reflection is the operating principle of automotive  rain sensors, which control automatic   windscreen/windshield wipers

115. Lenses 17 March 2017 119 [email protected]

116. Lenses A lens is defined as a portion of a refracting medium bordered by two curved surfaces which have a common axis. When each surface forms part of a sphere the lens is called a spherical lens. 17 March 2017 120 [email protected]

117. Sometimes, a spherical lens has a one plane surface, it is acceptable because a plane surface can be thought of as part of a sphere of infinite radius.

118. Spherical Lens Lens may be spherical (when each surface forms part of sphere, the lens is called a Spherical lens) where the concavity or convexity two different meridians are equal. 17 March 2017 122 [email protected]

119. Cylindrical Lens It may be cylindrical where there is unequal concavity in two meridians. The two meridians usually remains at right angels to each other and the less curved meridian being designed as axis of the lens. 17 March 2017 123 [email protected]

120. Lenses: (A–F), Spherical lenses: (A), biconvex; (B), biconcave; (C), planoconvex; (D), planoconcave; (E), concavoconvex, periscopic convex, converging meniscus; (F), convexoconcave, periscopic concave, diverging meniscus; (G, H), cylindrical lenses, concave and convex. 17 March 2017 124 [email protected]

121. 17 March 2017 125 [email protected]

122. Spherical Aberration The prismatic effect of the peripheral parts of the spherical lens causes spherical aberration. It was seen that the prismatic effect of a spherical lens is least in the paraxial zone and increases towards the periphery of the lens. 17 March 2017 126 [email protected]

123. Spherical Aberration Thus, rays passing through the periphery of the lens are deviated more than those passing through the paraxial zone of the lens.

124. Correction of Spherical Aberration Spherical aberration may be reduced by occluding the periphery of the lens by the use of “stops” so that only the paraxial zone is used. Lens form may also be adjusted to reduced spherical aberration, e,g plano-convex is better than biconvex. To achieve the best results, spherical surface must be 17 March 2017 128 [email protected]

125. Correction of Spherical Aberration abandoned and the lenses ground with aplantic surface, that the peripheral curvature is less than the central curvature. Another technique of reducing spherical aberration is to employ a doublet. This consists of a principal lens and a somewhat weaker lens of different R.I cemented together. 17 March 2017 129 [email protected]

126. Correction of Spherical Aberration The weaker lens must be of opposite power, and because it too has spherical aberration, it will reduce the power of the periphery of the principal lens more than the central zone. Usually, such doublets are designed to be both aspheric and achromatic.

127. A convex lens is thicker at the centre than at the edges. 17 March 2017 131 [email protected]

128. Image form by lens Unlike the mirrors, lenses have got two principal foci one on each side of the lens and the nodal point is situated within the substance of the lens just at the centre. If the image is situated on the other side of the object, it is called a Real Image and if it is on the same side it is called a Virtual Image. 17 March 2017 132 [email protected]

129. The point at which the principal plane and principal axis intersect is called the principal point or nodal point. Rays of light passing through the nodal point are undeviated. Light parallel to the principal axis is converged or diverged from the point F, the principal focus. 17 March 2017 133 [email protected]

130. Image form by lens For, an object in any position, the image can be constructed using two rays: 1) A ray from the top of the object which passes through the principal point/nodal point. 2) A ray parallel to the principal axis, which after refraction passes through (convex) or away from (concave) the second principal focus. 17 March 2017 134 [email protected]

131. 17 March 2017 135 [email protected]

132. Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens . When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus . The distance between the principal focus and the centre of the lens is called the focal length . 17 March 2017 136 [email protected]

133. 17 March 2017 137 [email protected]

134. Use of Convex Lenses Use of Convex Lenses – The Camera A camera consists of three main parts. The body which is light tight and contains all the mechanical parts. The lens which is a convex (converging) lens. The film or a charged couple device in the case of a digital camera. 17 March 2017 138 [email protected]

135. 17 March 2017 139 [email protected]

136. Use of Convex Lenses – The Camera The rays of light from the person are converged by the convex lens forming an image on the film or charged couple device in the case of a digital camera. The angle at which the light enters the lens depends on the distance of the object from the lens. If the object is close to the lens the light rays enter at a sharper angled. This results in the rays converging away from the lens. As the lens can only bend the light to a certain degree the image needs to be focussed in order to form on the film. This is achieved by moving the lens away from the film. 17 March 2017 140 [email protected]

137. Use of Convex Lenses – The Camera Similarly, if the object is away from the lens the rays enter at a wider angle. This results in the rays being refracted at a sharper angle and the image forming closer to the lens. In this case the lens needs to be positioned closer to the film to get a focused image. Thus the real image of a closer object forms further away from the lens than the real image of a distant object and the action of focusing is the moving of the lens to get the real image to fall on the film. The image formed is said to be real because the rays of lighted from the object pass through the film and inverted (upside down). 17 March 2017 141 [email protected]

138. The Magnifying Glass A magnifying glass is a convex lens which produces a magnified (larger) image of an object. A magnifying glass produces an upright, magnified virtual image. The virtual image produced is on the same side of the lens as the object. For a magnified image to be observed the distance between the object and the lens must be shorter than the focal length of the lens. 17 March 2017 142 [email protected]

139. For a magnified image to be observed the distance between the object and the lens has to be shorter than the focal length of the lens. The image formed is upright, magnified and virtual. 17 March 2017 143 [email protected]

140. 17 March 2017 144 [email protected] Magnification : The magnification of a lens can be calculated using the following formula;

141. 17 March 2017 145 [email protected]

142. Aspheric lens An aspheric lens or asphere is a Lens whose surface profiles are not portions of a sphere or cylinder. The asphere's more complex surface profile can reduce or eliminate spherical aberration and also reduce other optical aberration compared to a simple lens. 17 March 2017 146 [email protected]

143. PHYSICAL PROPERTIES OF LIGHT 1) Polarization 2) Interference 3) Diffraction 4) Superimposition

144. Polarization Since a light wave’s electric field vibrates in a direction perpendicular to its propagation motion, it is called a transverse wave and is polarizable. A sound wave, by contrast, vibrates back and forth along its propagation direction and thus is not polarizable. 17 March 2017 148 [email protected]

145. What is Polarization? Light waves are travelling may or may not be parallel to each other. If directions are randomly related to each other the light is UNPOLARIZED/ NONPOLARIZED . If parallel to each other is called POLARIZED . 17 March 2017 149 [email protected]

146. Non polarized light NON POLARIZED LIGHT 17 March 2017 150 [email protected]

147. Polarized light POLARIZED LIGHT 17 March 2017 151 [email protected]

148. Polarized light 17 March 2017 152 [email protected]

149. How light is polarized? Polarized light is produced from ordinary light by an encounter with a polarizing substances or agent. Polarizing substances, e,g. calcite crystal, only transmit light rays which are vibrating in one particular plane. Thus only a proportion of incident light is transmitted onward and the emerging light is polarized. 17 March 2017 153 [email protected]

150. How light is polarized? A polarizing medium reduces radiant intensity but does not affect spectral composition. In nature, light is polarized on reflection from a plane surface. Such as water, if the angle of incidence is equal to the polarizing angle for the substances. The polarizing angle is dependent on the refractive index of the substance. 17 March 2017 154 [email protected]

151. Application of polarized light Polarized sunglasses to exclude selectively the reflected horizontal polarized light. Such glasses are of great use in reducing glare from the sea or wet roads. Instruments: (to reduced reflected glare from the cornea) example: Slit lamp Ophthalmoscope 17 March 2017 155 [email protected]

152. Application of polarized light Binocular vision polarizing glass – May be used to dissociate the eyes i,e in Titmus test Also used in pleoptic to produced Haidinger’s brushes and in optical lens making to examine lens for stress. 17 March 2017 156 [email protected]

153. Birefringence Some substances have double refractive index though they transmit light into 2 direction and they are called Birefringence A widely used birefringent material is Calcite Its birefringence is extremely large, with indices of refraction for the o- and e-rays of 1.6584 and 1.4864 respectively. 17 March 2017 [email protected] 157

154. Calcite Crystal 17 March 2017 [email protected] 158

155. Applications of Birefringence Birefringence finds use in the following applications: Polarizing prisms and retarder plates Liquid crystal displays Medical Diagnostics 17 March 2017 [email protected] 159

156. 2. Interference Before discussing interference we should have clear idea about wave properties of light.

157. Picture of a light wave 3/17/2017 161 [email protected]

158. The maximum value of the wave displacement is called the amplitude ( A) of the wave. The cycle starts at zero and repeats after a distance. This distance is called the wavelength (λ). Light can have different wavelengths. The inverse of the wavelength (1/λ) is the wave number (ν), which is expressed in cm–1. 3/17/2017 162 [email protected]

159. The wave propagates at a wave speed (v). This wave speed in a vacuum is equal to c, and is less than c in a medium. At a stationary point along the wave, the wave passes by in a repeating cycle. The time to complete one cycle is called the cycle time or period 3/17/2017 163 [email protected]

160. Another important measure of a wave is its frequency ( f). It is measured as the number of waves that pass a given point in one second. The unit for frequency is cycles per second, also called hertz ( Hz ). 3/17/2017 164 [email protected]

161. As we can see, the frequency and the period are reciprocals of one another. If the wave speed and wavelength are known, the frequency can be calculated .

162. Wave like model of Light The particle-like model of light describes large-scale effects such as light passing through lenses or bouncing off mirrors . However, a wavelike model must be used to describe fine-scale effects such as interference and diffraction that occur when light passes through small openings or by sharp edges. The propagation of light or electromagnetic energy through space can be described in terms of a traveling wave motion. 3/17/2017 166 [email protected]

163. The wave moves energy—without moving mass—from one place to another at a speed independent of its intensity or wavelength. This wave nature of light is the basis of physical optics and describes the interaction of light with media. Many of these processes require calculus and quantum theory to describe them rigorously. 3/17/2017 167 [email protected]

164. Characteristics of light waves To understand light waves, it is important to understand basic wave motion itself. Water waves are sequences of crests (high points) and troughs (low points) that “move” along the surface of the water. When ocean waves roll in toward the beach, the line of crests and troughs is seen as profiles parallel to the beach. An electromagnetic wave is made of an electric field and a magnetic field that alternately get weaker and stronger. 3/17/2017 168 [email protected]

165. Characteristics of light waves The directions of the fields are at right angles to the direction the wave is moving, just as the motion of the water is up and down while a water wave moves horizontally. 3/17/2017 169 [email protected]

166. 2. Interference When two light waves from different coherent sources meet together, then the distribution of energy due to one wave is disturbed by the other. This modification in the distribution of light energy due to super- position of two light waves is called "Interference of light" 17 March 2017 [email protected] 170

167. Conditions for Interference  The two sources of light should emit continuous waves of same wavelength and same time period i.e. the source should have phase coherence. The two sources of light should be very close to each other. The waves emitted by two sources should either have zero phase difference or no phase difference. 17 March 2017 [email protected] 171

168. 17 March 2017 [email protected] 172

169. Coherent sources Those sources of light which emit light waves continuously of same wavelength, and time period,    frequency and amplitude and have zero phase difference or constant phase difference are coherent    sources. 17 March 2017 [email protected] 173

170. Types of interference There are two types of interference. Constructive interference.   Destructive interference 17 March 2017 [email protected] 174

171. Interference 17 March 2017 [email protected] 175 constructive interference destructive interference

172. Interference 17 March 2017 [email protected] 176 Resultant of constructive interference Resultant of destructive interference constructive interference destructive interference

173. constructive interference   When two light waves superpose with each other in such away that the crest of one wave falls on the crest of the second wave, and trough of one wave falls on the trough of the second wave, then the    resultant wave has larger amplitude and it is called constructive interference 17 March 2017 [email protected] 177

174. destructive interference When two light waves superpose with each other in such away that the crest of one wave coincides the    trough of the second wave, then the amplitude of resultant wave becomes zero and it is called   destructive interference . 17 March 2017 [email protected] 178

175. Diffraction The term diffraction, from the Latin diffringere , 'to break into pieces', referring to light breaking up 17 March 2017 [email protected] 179

176. Concept of diffraction Diffraction is the bending of waves around obstacles, or the spreading of waves by passing them through an aperture, or opening. Any type of energy that travels in a wave is capable of diffraction, and the diffraction of sound and light waves produces a number of effects. 17 March 2017 [email protected] 180

177. Concept of diffraction 17 March 2017 [email protected] 181 Diffraction of light waves, is much more complicated, and has a number of applications in science and technology, including the use of diffraction gratings in the production of holograms .

178. Diffraction of light 17 March 2017 [email protected] 182

179. Observing Diffraction in Light Wavelength of light plays a role in diffraction; so, too, does the size of the aperture relative to the wavelength. Hence, most studies of diffraction in light involve very small openings, as, for instance, in the diffraction grating. But light does not only diffract when passing through an aperture, it also diffracts around obstacles. 17 March 2017 [email protected] 183

180. Observing Diffraction in Light When light passes through an aperture, most of the beam goes straight through without disturbance, with only the edges experiencing diffraction. If, however, the size of the aperture is close to that of the wavelength, the diffraction pattern will widen. when light is passed through extremely narrow openings, its diffraction is more noticeable. 17 March 2017 [email protected] 184

181. Diffraction Grating A diffraction grating is an optical device that consists of not one but many thousands of apertures: Rowland's machine used a fine diamond point to rule glass gratings, with about 15,000 lines per in (2.2 cm). Diffraction gratings today can have as many as 100,000 apertures per inch. 17 March 2017 [email protected] 185

182. The apertures in a diffraction grating are not mere holes, but extremely narrow parallel slits that transform a beam of light into a spectrum. Each of these openings diffracts the light beam, but because they are evenly spaced and the same in width, the diffracted waves experience constructive interference.

183. This constructive interference pattern makes it possible to view components of the spectrum separately, thus enabling a scientist to observe characteristics ranging from the structure of atoms and molecules to the chemical composition of stars. 17 March 2017 [email protected] 187

184. You may also notice that the light is alternately bright and dark as you look through the curtain. This is from  interference . The bright places are where light waves are adding together. The dark places are where the waves cancel. With visible light, interference always occurs with diffraction. 17 March 2017 [email protected] 188

185. 17 March 2017 [email protected] 189