01 properties of light

These lecture has prepared for postgraduate student (Ophthalmology) according to the curriculum of Bangladesh College of Physician and Surgeons (BCPS) and Bangabondhu Sheikh Mujib Medical University (BSMMU) Bangladesh

1. OPTICS: BASICS CONCEPTS Md Anisur Rahman (Anjum) Professor & Head (Ophthalmology) Dhaka Medical College, Dhaka Contact: 880-1711-832397 [email protected]

2. What is optical science? AND What is light? In physics the term more broadly refers to   “the study of the behavior of light and its interactions with matter” . In short, “light can be defined as energy to which the human eye is sensitive”. ( A.R.Elkington . P: 01)

3. Theory of light There are four common theories to describe nature of light: Newton’s corpuscular theory Huygen’s wave theory: Maxwell’s electromagnetic theory Einstein quantum theory Aug 28, 20198/28/2019 [email protected] 3

4. Newton’s corpuscular theory It is based on the following points 1. Light consists of very tiny particles known as “corpuscular”. 2. These corpuscles on emission from the source of light travel in straight line with high velocity Aug 28, 20198/28/2019 [email protected] 4

5. Newton’s corpuscular theory 3. When these particles enter the eyes, they produce image of the object or sensation of vision. 4. Corpuscles of different colour have different sizes. Aug 28, 20198/28/2019 [email protected] 5

6. Huygen’s wave theory of light In 1679, Christian Huygens proposed the wave theory of light. According to Huygen’s wave theory: 1. each point in a source of light sends out waves in all directions in hypothetical medium called "ETHER 2. Light is a form of energy Aug 28, 20198/28/2019 [email protected] 6

7. Huygen’s wave theory of light 3. Light travels in the form of waves 4. A medium is necessary for the propagation of waves & the whole space is filled with an imaginary medium called Ether 5. Light waves have very short wave length Aug 28, 20198/28/2019 [email protected] 7

8. Picture of a light wave 6/2/2020 8 [email protected]

9. Maxwell’s electromagnetic Theory In 1873, Maxwell proposed that light is not a mechanical wave but electromagnetic in character i.e it consists of electric and magnetic fields travelling freely through vacuum. This theory explains most of the phenomena related to light satisfactorily but has partial success in explaining scattering and fails completely to explain the photoelectric effect of light Aug 28, 20198/28/2019 [email protected] 9

10. Einstein  quantum theory of light In 1905 ,  Einstein   created the quantum theory of light , the idea that  light  exists as tiny packets, or particles, which he called photons. ... Later in 1905 came an extension of special relativity which  Einstein  proved that energy and matter are linked in the most famous relationship in physics: E=mc 2 .   E=mc 2 .   (The energy content of a body is equal to the mass of the body times the speed of light squared).

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 2. Physical optics 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:

18. Picture of a light wave 6/2/2020 20 [email protected]

19. 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. 6/2/2020 21 [email protected]

20. 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 6/2/2020 22 [email protected]

21. 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). 6/2/2020 23 [email protected]

22. 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.

23. 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. 6/2/2020 25 [email protected]

24. 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. 6/2/2020 26 [email protected]

25. 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. 6/2/2020 27 [email protected]

26. 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. 6/2/2020 28 [email protected]

27. 2. Physical optics 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.

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

29. So what are the properties of light? Reflection Refraction Interference Polarisation Diffraction Dispersion

30. Electromagnetic spectrum: optical radiation: colour Aug 28, 20198/28/2019 [email protected] 32

31. Electromagnetic spectrum: optical radiation: colour Optical radiation lies between X- rays and microwaves in the electromagnetic spectrum, and is divisible into 7 wavebands. Each of these 7 wavebands group together wavelength which elicit similar biological reactions The 7 domains are: Aug 28, 20198/28/2019 [email protected] 33

32. The 7 domains are Ultraviolet C (UV – C): 200 – 280 nanometers. Ultraviolet B (UV – B): 280 – 315 nanometers. Ultraviolet A (UV – A): 315 – 400 nanometers. Visible radiation: 400 – 780 nm Infrared A (IRA): 780 – 1400 nm Infrared B (IRB) 1400 – 3000 nm Infrared C (IRC) 3000 – 10000 nm Aug 28, 20198/28/2019 [email protected] 34

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35. The normal eye is able to discriminate between light of shorter or longer wave length within the visible spectrum by means of colour sense originating from three different classes of cone cells into retina. Shorter the wavelength greater the energy Aug 28, 20198/28/2019 [email protected] 37

36. Cornea & Sclera absorb essentially all the incident optical radiation at very short wavelengths in the UV – B & C. And long wavelength in the infrared (IR – B, & IR – C). LENS absorb the UV- A Aug 28, 20198/28/2019 [email protected] 38

37. The wavelength 400 – 1400 passes through the ocular media to fall on the RETINA. The visible wavelengths stimulate the retinal photoreceptor & giving sensation of light. Near IR causes thermal effect to retina. Aug 28, 20198/28/2019 [email protected] 39

38. The visible wavelengths of the electromagnetic spectrum are between 400 nm & 780 nm, The colour of any object is determined by the wavelength emitted or reflected from the surface. White colour is the mixture of wavelengths of the visible spectrum. Aug 28, 20198/28/2019 [email protected] 40

39. Colour is perceived by three population of cone photoreceptor in the retina which are sensitive to light of Short (BLUE). M iddle (GREEN). L ong (RED) wavelengths of the visible spectrum. Aug 28, 20198/28/2019 [email protected] 41

40. Explain: Visual field loss in glaucoma is detected earlier if perimetry is performed using a blue light stimulus on a yellow background. Explain: Why there is macular burn during solar eclipse? Aug 28, 20198/28/2019 [email protected] 42

41. Acquired optic nerve disease tends to cause red – green defects. An exception occurs in glaucoma and in AD optic neuropathy which initially causes a predominantly blue – yellow defect; it has been recently found that visual field loss in glaucoma is detected earlier if perimetry is performed using a blue light stimulus on a yellow background. Aug 28, 20198/28/2019 [email protected] 43

42. Acquired retinal disease tends to cause blue – yellow defects (except in cone dystrophy and Stargardt’s disease, which cause a predominantly red – green defect,) Aug 28, 20198/28/2019 [email protected] 44

43. Different properties of light 1. INTERFERENCE Aug 28, 20198/28/2019 [email protected] 45

44. Interference: What to read? a) Some basic definitions: C oherent sources. Conditions for coherent sources. Interference. Conditions for Interference Types of Interference Practical application of interference

45. What is coherent sources? C oherent sources two waves are said to be coherent , if they emit same frequency or wave length and are in phase or constant phase difference.

46. C onditions for obtaining coherent source: C oherent sources are obtained from single source. T he source must emit mono chromatic light. T he path difference between light sources must be very small.

47. Sunlight (Many different colours) LED One colour (Monochromatic) and waves not in phase (non-coherent) LASER: One colour (Monochromatic) and waves in phase (coherent )

48. Why can’t two sources behave as coherent sources? Two different sources can never produce waves of same phase because each source of light contains infinite number of atoms and the waves which are emitted by them will not be in phase. The atoms after absorbing energy go to excited states and emit radiations when fall back to ground state.

49. Interference: Definition Interference The phenomenon of redistribution of energy due to super position of light waves from two coherent sources is called interference.

50. 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. Aug 28, 20198/28/2019 [email protected] 52

51. Types of interference CONSTRUCTIVE INTERFERENCE: In constructive interference the amplitude of the resultant wave is greater than that of either individual wave. DESTRUCTIVE INTERFERENCE: In destructive interference the amplitude of the resultant wave is less than that of either individual wave .

52. Types of interference There are two types of interference. Constructive interference.   Destructive interference Aug 28, 20198/28/2019 [email protected] 54

53. Interference Aug 28, 20198/28/2019 [email protected] 55 constructive interference destructive interference

54. Interference Aug 28, 20198/28/2019 [email protected] 56 Resultant of constructive interference Resultant of destructive interference constructive interference destructive interference

55. 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 Aug 28, 20198/28/2019 [email protected] 57

56. 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 . Aug 28, 20198/28/2019 [email protected] 58

57. Practical application of Interference Y oung’s double slit experiment. E xpression for fringe width. S ustained interference C onditions for sustained interference

58. Practical application of Interference Young’s Double Slit Experiment Thomas Young first demonstrated interference in light waves from two sources in 1801. The narrow slits S1 and S2 act as sources of waves. The waves emerging from the slits originate from the same wave front and therefore are always in phase .

59. Resulting Interference pattern The light from the two slits forms a visible pattern on a screen. The pattern consists of a series of bright and dark parallel bands called fringes. Constructive interference occurs where a bright fringe occurs. Destructive interference results in a dark fringe .

60. Different properties of light 2 . Polarization Aug 28, 20198/28/2019 [email protected] 63

61. 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. Aug 28, 20198/28/2019 [email protected] 64

62. 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. Aug 28, 20198/28/2019 65 [email protected]

63. Non polarized light NON POLARIZED LIGHT Aug 28, 20198/28/2019 66 [email protected]

64. Polarized light POLARIZED LIGHT Aug 28, 20198/28/2019 67 [email protected]

65. Polarized light Aug 28, 20198/28/2019 68 [email protected]

66. 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. Aug 28, 20198/28/2019 69 [email protected]

67. 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. Aug 28, 20198/28/2019 70 [email protected]

68. 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. Aug 28, 20198/28/2019 71 [email protected]

69. Application of polarized light Instruments: (to reduced reflected glare from the cornea) Slit lamp Ophthalmoscope Aug 28, 20198/28/2019 [email protected] 72

70. 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. Aug 28, 20198/28/2019 73 [email protected]

71. Different properties of light 3. Diffraction Aug 28, 20198/28/2019 [email protected] 74

72. Diffraction The term diffraction, from the Latin diffringere, 'to break into pieces', referring to light breaking up Aug 28, 20198/28/2019 [email protected] 75

73. 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. Aug 28, 20198/28/2019 [email protected] 76

74. Concept of diffraction Aug 28, 20198/28/2019 [email protected] 77 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 .

75. Hologram: a  special type of photograph or image  made with a laser in which the objects shown look solid, as if they are real, rather than  flat. In short, it is 3 dimensional image.

76. Properties of light 4. Dispersion of light

77. Physical properties of light 4. D i s p e r s ion Aug 28, 20198/28/2019 [email protected] 80

78. Dispersion of Light? The light rays from the sun consist of seven different colors – red, orange, yellow, green, blue, indigo and violet (ROYGBIV) . We see seven different colors when these rays are passed through a glass prism. The splitting of a ray into its component colors is known as dispersion of light . The band of colors into which the light splits is known as a spectrum. 6/2/2020 81 [email protected]

79. Photons of different wavelength travel with different speed. The red photon has the longest wavelength and the violet photon has the shortest wavelength. Thus when a white light passes through a prism, different photons cross the medium at different speeds and deviating by different angles. 6/2/2020 82 [email protected]

80. The red color appears at the top of the spectrum because it is bent the least or it is refracted the least.. On the other hand, the violet end of the spectrum is bent the most or refracted most, as it takes longer to traverse the glass medium. 6/2/2020 83 [email protected]

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