The Nature and Propagation of Light

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The Nature and Propagation of Light

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1. The Nature and Propagation of Light

2. IN THE BEGINNING - (4.5 Billion BC) In the beginning it was dark and cold. There was no sun, no light, no earth, no solar system. Then slowly, about 4.5 billion years ago, a swirling nebula, - a huge cloud of gas and dust was formed.

3. THE SUN - (4 Billion BC) Eventually this cloud contracted and grew into a central molten mass that became our sun. At first the sun was a molten glow. As the core pressure increased, and the temperature rose to millions of degrees - a star was born. Through the process of thermonuclear hydrogen fusion, the sun began to shine. This was the nebular hypothesis, first proposed in 1755 by the great German philosopher, Immanuel Kant.

4. THE EARTH - (4 Billion BC) Soon after the Sun was formed, the Earth and our other planets were formed from violent explosions and spinoffs from the process that created the Sun. The nine planets created are now known as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. As rocks and other particles collided forming the Earth, it became molten. The surface of the Earth cooled and hardened.

5. EARLY LIFE - (3 Billion BC) Gradually oceans appeared and sunlight and water gave birth to life, eventually, intelligent life. Without light, there would be no life. Life was dependent on three things being present: a.) the basic long molecule building block, carbon, b.) water, and c.) light.

6. Evidence of life The oldest verified evidence of life comes from Rhodesia, where rocks formed approximately 3 billion years ago, bear 'stromatolites', the fossilized remains of algae.

7. LIGHT AND THE BIBLE There are more than 200 references to the word 'light' in the Bible. About 75 of these occur in the new testament. Light was the first of God's creations, according to the book of Genesis. "And God said, let there be light, and there was light". (Old Testament, Genesis, i,3.)

8. Nature of Light-the history and classical theories Particle theory (Newton): Until the middle of seventeenth century, light was generally thought to consist of a stream of some sort particles or corpuscles. Wave theory (Huygens): Early in the nineteenth century, evidence for the wave theory become more and more persuasive. The experiments of Tomas Young Maxwell’s theory (1873) and Heinrich Hertz’s experiments.

9. The nature of light-Quantum theory By the end of nineteenth century, researchers believed that little if any would be added in the future to our knowledge of nature of light. Photoelectric emission: Einstein’s work on photoelectric effect (1905). Compton effect (1921). Quantum electrodynamics (1930).

10. Sources of light Thermal radiation: all bodies emit electromagnetic radiation as a result of thermal motion of their molecules. Carbon arc lights: typical temperature 4000°C, often used in projectors, searchlights and lighthouse Light sources using arc discharge in a metal vapor. Mercury and sodium arc lamps. Fluorescent lamps Laser (light amplification by stimulated emission of radiation).

11. The speed of light Speed of light in free space: 3x108m/s. Roemer’s (Danish astronomer) measurement by observing the eclipse of Jupiter’s satellite (1676). 2.1x108m/s. Fizeau’s (French scientist 1849) measurement: c= 3.15x108m/s C= 2.9979246x108 ±2m/s

12. ROEMER, OLAF - (1644 - 1710) The speed of light was roughly calculated in 1675 by the Danish astronomer Olaf Roemer. Roemer's submitted his work to the French Academy of Science in 1675. He was ridiculed and his work was largely forgotten. Fifteen years after his death the British astronomer James Bradely started out from the same observations that R”emer had made and his thinking ultimately led to a conclusive figure for the speed of light (186,000 miles per second)

13. Reflection and refraction The incident, reflected, and refracted rays, and the normal to the surface, all lie in the same plane. The law of reflection: The angle of reflection is equal to the angle of incidence, the incident, reflected light and the normal all lie in the same plane. The Law of refraction: For monochromatic light, the ratio of the sine of the incident angle and that of the refraction angle is a constant; the incident, refracted and the normal all lie in the same plane.

14. More about reflection and refraction The intensity of reflected and refracted light depend on the angle of incidence: the fraction reflected light is smallest at normal incidence. The passage of a ray of light in going from one medium to another is reversible.

15. The index of refraction The index of refraction (or the refractive index) for a given medium: the ratio of sine of incident angle in vacuum and the sine of refraction angle in the medium. Refractive index depends on wavelength. If no wavelength is stated, the index is usually assumed to be that corresponding to the yellow light of wavelength 589nm.

16. Total internal reflection When a ray of light in medium A of index na strike the surface of second medium b of index nb, where na>nb, there is a incident angle for which the refracted ray emerges tangent to the surface. This incident angle is called critical angle. Beyond the critical angle, the ray does not pass into the second medium but is totally internally reflected.

17. The application of total internal reflection Using total internal reflection instead of metallic surfaces as reflector. Optical fiber.

18. Questions What is the maximum speed? Why the speed of light is the maximum speed? Can you prove it experimentally? Is there any evidence that the speed of light is not the maximum speed? What would happen if speed of light is not the upper limit of speed?

19. Huygen’s principle Every point of wave front may be considered the source of secondary wavelets, which spread out in all directions with a speed equal to the propagation speed of waves.

20. Dispersion The dependence of wave speed on wavelength is called dispersion. The dispersion of waves with longer wavelength is smaller than that of shorter wavelengths.

21. Interference and diffraction Physical optics. Geometrical optics is not enough in discussing interference and diffraction. One has to treat light as wave motion. Monochromatic light-an unattainable idealization. The bright green line in the mercury spectrum has an average wavelength of 546.1nm, with a spread of wavelength of the order of ±0.001nm, depending on the temperature and pressure of the mercury vapor in the lamp. Helium-neon laser emits light at 632.8 nm with a line width of ±0.000001 nm.

22. Interference refers to any situation in which two or more waves overlap in space. The principle of linear superposition: when two or more waves overlap, the resulted displacement at any point and at any instant may be found by adding the instantaneous displacements that would be produced at the point by the individual waves if each of them were present alone.

23. Constructive interference. It occurs whenever the path difference for the two sources is an integer multiple of the wavelength. Destructive interference. It occurs whenever the path difference for the two sources is half-integer number of the wavelength. Coherent sources: sources derived from a single source and having a definite phase relation.

24. Young’s experiment

25. YOUNG, THOMAS - (1773 - 1829) Thomas Young (born in England) was a London physician, linguist, and expert in many fields of science. He read fluently at the age of two (2) At an age of fourteen (14) he was familiar with Latin, Greek, Hebrew, Arabic, Persian, French, and Italian. Young strongly supported the [HUYGENS] wave theory of light, mainly by virtue of his now famous double slit experiment (1801) demonstrating the interference of light waves. He was also the first to describe and measure astigmatism (1801).

27. If d is the distance between the two slits and ? the angle between line PA and OA, constructive interference occurs when

28. If point P is at the centre of mth fringe, the distance y from the zeroth to the mth fringe is y=R tan ?˜R sin?=Rm?/d The wavelength may be found by measuring R,m,d and y

29. Homework Find two method to measure the wavelength of light.

30. Interference in thin films Two plates of glass separated by a wedge of air:

31. A half-cycle phase change occurs whenever the material in which the wave is initially traveling before reflection has a smaller refractive index than the second material forming the interface. Example Suppose the two glass plates are 10 cm long and are in contact at one end and are separated at the other end by a height of h=0.02 mm. What is the spacing of the resulting interference fringes? Is the fringe adjacent to the line of contact bright or dark? Assume ?= 500 nm.

32. Solution The fringe at the line of contact is dark due to the half-cycle phase shift at the lower surface of the air wedge. The condition for destructive interference is now 2d=m? with m=0,1,2…From similar triangles, d is proportional to the distance x from the line of contact: d/x=h/l

33. 2xh/l=m? Or X=ml?/2h=m(1.25 mm) The successive dark fringes are spaced 1.25 mm apart. Further question: If the space between the plates is water (n=1.33) instead of air, the phase changes are the same but the wavelength is ?=500/n=376 nm.

34. Thin coatings on glass Nonreflextive coatings for glass: a thin layer of hard transparent material with an index of refraction smaller than that of the glass is deposited on the surface of the glass. If the film thickness is ¼ wavelength in the film and assume normal incidence, complete destructive interference will result.

35. The Michelson interferometer

36. Application of Michelson interferometer Precise measurement of wavelength. The meter is defined as a length equal to 1650763.73 wavelengths of the orange-red light of krypton 86. Michelson-Morley experiment (1887).It attempted to measure the relative motion of the earth and the ether by measuring the change of light in directions parallel and perpendicular to the earth’s motion. The results disproved the existence of the ether, which contradicted the Newtonian physics but was explained by Einstein’s special relativity (1905).

37. Diffraction Diffraction may be defined as the bending of light around an obstacle. Fresnel diffraction: light source and the screen are at large but finite distance from the obstacle forming the diffraction pattern.

38. Fraunhofer diffraction: the light source is far enough away so that the diffraction pattern appears on a screen in the second focal plane of the lens. The essential features observed in diffraction effects can be predicted by using Huygens principle together with the principle of interference.

39. The condition for a dark fringe in Fraunhofer single slit diffraction with n=1,2,3…: The diffraction grating: a very large number of parallel slits, all of the same width and spaced at regular intervals. Condition for a maximum is with m=0,1,2,…

40. Example 1 The wavelengths of the visible light are approximately in the range from 400nm to 700nm. Find the angular breadth of the first-order visible spectrum produced by a plane grating having 6000 lines per cm, when light is incident normally on the grating. Solution: The grating spacing d=1/600000=1.67x10-6m. The angular deviation of the violet is

41. The angular deviation of the red is Hence, the first order visible spectrum includes an angle of 24.3-13.9=10.9 degree.

42. Diffraction of x-rays by a crystal X-rays were discovered by Roentgen in 1895. It’s wavelength is of the order of 10-8 cm. Laue pointed out in 1913: If the atoms in a crystal were arranged in a regular way, a crystal might serve as a three dimensional diffraction grating for x-rays. The experiment were performed successfully by Friederich and Knipping.

43. Discovery of DNA structure and x-ray diffraction On 25 April 1953 James Watson and Francis Crick, working in a small Medical Research Council unit in the Cavendish Laboratory, Cambridge, published a short letter in Nature. It described a remarkable two-chain helical structure for DNA. The two foundation stones of modern biology and medicine - DNA structure and protein structure – were recognized in 1962 by the award of a Nobel Prize in Physiology to Watson, Crick and Wilkins.

44. Rosalind Franklin

45. Franklin was responsible for much of the research and discovery work that led to the understanding of the structure of deoxyribonucleic acid, DNA. James Watson, Francis Crick, and Maurice Wilkins received a Nobel Prize for the double-helix model of DNA in 1962, four years after Franklin's death at age 37 from ovarian cancer.

46. The story of DNA and Rosalind Franklin Franklin attended one of the few girls' schools in London that taught physics and chemistry. When she was 15, she decided to become a scientist. Her father was decidedly against higher education for women and wanted Rosalind to be a social worker. In 1938 she enrolled at Newnham College, Cambridge, graduating in 1941. Doctorate in physical chemistry, which she earned from Cambridge University in 1945.

47. After Cambridge, she spent three productive years (1947-1950) in Paris where she learned X-ray diffraction techniques. In 1951, she returned to England as a research associate in John Randall's laboratory at King's College, London. It was in Randall's lab that she crossed paths with Maurice Wilkins. She and Wilkins led separate research groups and had separate projects, although both were concerned with DNA.

48. Wilkins was away at the time, and when he returned he misunderstood her role, behaving as though she were a technical assistant. Both scientists were actually peers. His mistake, acknowledged but never overcome, was not surprising given the climate for women at the university then. Only males were allowed in the university dining rooms.

49. Franklin made marked advances in x-ray diffraction techniques with DNA. She adjusted her equipment to produce an extremely fine beam of x-rays. She extracted finer DNA fibers than ever before and arranged them in parallel bundles. And she studied the fibers' reactions to humid conditions. All of these allowed her to discover crucial keys to DNA's structure. Wilkins shared her data, without her knowledge, with James Watson and Francis Crick, at Cambridge University, and they pulled ahead in the race, ultimately publishing the proposed structure of DNA in March, 1953.

50. Franklin's Photo 51

51. She was able to get this remarkable image—the clearest image of DNA ever created up until that time—with her advanced techniques of X-ray diffraction. Using Franklin's image as physical evidence, Watson and Crick then went on to publish their Nobel Prize-winning theoretical structure of DNA in Nature in 1953.

56. What can we learn from the story of DNA In the early 1950s, the race to discover DNA was on. At Cambridge University, graduate student Francis Crick and research fellow James Watson (b. 1928) had become interested. Meanwhile at King's College in London, Maurice Wilkins (b. 1916) and Rosalind Franklin were also studying DNA. The Cambridge team's approach was to make physical models to create an accurate picture of the molecule. The King's team took an experimental approach, looking particularly at x-ray diffraction images of DNA.

57. Innovation!!! Choose an important scientific problem to work on when you are young. Using advanced technology to different research fields. Publish research results timely. Exchange and share information between experimental and theoretical scientists. DNA's discovery has been called the most important biological work of the last 100 years, and the field it opened may be the scientific frontier for the next 100.

58. Holography Holography is a technique for recording and reproducing an image of an object without the use of lenses or mirrors. A holographic image is truly three dimensional and can be viewed from different directions to reveal different sides.

59. The basic procedure to make holography The object is illuminated by monochromatic light and a photographic film is located so that it is struck by scattered light from the object and also by direct light from the source. Interference between the direct and scattered light leads to the formation and recording of a complex interference pattern on the film.

60. To form the images, one simply projects laser light through the developed film. Two images are formed, a virtual image on the side of the film nearer to the light source, and a real image on the opposite side.

61. Reasons why the English language is so difficult to learn: 1) The bandage is wound around the wound 2) The farm is used to produce produce 3) The dump was so full it had to refuse more refuse 4) We must polish the Polish furniture 5) He could lead if he could get the lead out. 6) The soldier decided to desert his dessert in the desert. 7) There is no time like the present to present the present. 8) The bass was painted on the head of the bass drum 9) When shot at, the dove dove into the bushes

62. 10) I did nor object to the object 11) The insurance was invalid for the invalid 12) There was a row among the oarsmen about how to row 13) They were too close to the door to close it 14) The buck does funny things when the does are present 15) A seamstress and a sewer fell down into a sewer 16) To help with the planting, the farmer taught his sow to sow 17) The wind was too strong to wind the sail 18) After a number of injections my jaw got number 19) Upon seeing the tear in the painting I shed a tear 20) I had to subject the subject to a series of tests 21) How can I intimate this to my most intimate friend?

63. There is no egg in eggplant nor ham in hamburger; neither apple nor pine in pineapple. English muffins were not invented in England, nor French Fries in France. Sweetmeats are candies while sweetbreads, which aren't sweet, are meat.

64. We take English for granted. But if we explore it's paradoxes we find that quicksand can work slowly, boxing rings are square and a guinea pig is neither from Guinea nor is it a pig. And why is it that writers write but fingers don't fing, grocers don't groce and hammers don't ham? If the plural of tooth is teeth, why isn't the plural of booth beeth? One goose, 2 geese. So one moose 2 meese? One index, 2 indices? Doesn't it seem crazy that you can make amends but not one amend? If you have a bunch of odds and ends and get rid of all but one of them, what do you call it? If teachers taught, why didn't' preachers praught? If a vegetarian eats vegetables, what does a humanitarian eat?

65. Sometimes I think all English speakers should be committed to an asylum for the verbally insane. In what language do people recite at a play and play at a recital? Ship cargo by truck and send cargo by ship? Have noses that run and feet that smell? How can a slim chance and a fat chance be the same things, while a wise man and a wise guy are opposites? You have to marvel at the unique lunacy of a language in which your house can burn down as it burns up, in which you fill in a form by filling it out, and in which an alarm goes off by going on. English was invented by people not computers and it reflects the creativity of the human race ( which isn't a race at all). That is why when the stars are out, they are visible, but when the lights are out the are invisible.

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