The Nature and Propagation of Light

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The Nature and Propagation of Light
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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.

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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.

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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.

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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.

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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.

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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.)

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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
• Maxwells theory (1873) and Heinrich Hertzs
  experiments.

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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 Einsteins work on
  photoelectric effect (1905).
• Compton effect (1921).
• Quantum electrodynamics (1930).

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Sources of light

• Thermal radiation all bodies emit
  electromagnetic radiation as a result of thermal
  motion of their molecules.
• Carbon arc lights typical temperature 4000C,
  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).

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The speed of light

• Speed of light in free space 3x108m/s.
• Roemers (Danish astronomer) measurement by
  observing the eclipse of Jupiters satellite
  (1676). 2.1x108m/s.
• Fizeaus (French scientist 1849) measurement c
  3.15x108m/s
• C 2.9979246x108 2m/s

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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 Remer had made and his thinking ultimately
  led to a conclusive figure for the speed of light
  (186,000 miles per second)

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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.

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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.

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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.

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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 nagtnb, 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.

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The application of total internal reflection

• Using total internal reflection instead of
  metallic surfaces as reflector.
• Optical fiber.

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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?

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Huygens 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.

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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.

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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.

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• 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.

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• 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.

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Youngs experiment
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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).

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P
y
s1
?
d
O
A
s2
R
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• If d is the distance between the two slits and ?
  the angle between line PA and OA, constructive
  interference occurs when

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

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Homework

• Find two method to measure the wavelength of
  light.

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Interference in thin films

• Two plates of glass separated by a wedge of air

h
d
x
l
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• 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 h0.02
  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.

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• 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 2dm? with
  m0,1,2From similar triangles, d is proportional
  to the distance x from the line of contact
• d/xh/l

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• 2xh/lm?
• Or
• Xml?/2hm(1.25 mm)
• The successive dark fringes are spaced 1.25 mm
  apart.
• Further question
• If the space between the plates is water (n1.33)
  instead of air, the phase changes are the same
  but the wavelength is ?500/n376 nm.

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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.

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The Michelson interferometer
M2
Moveable mirror
M1
Light source
(Fixed mirror)
Beam splitter
Compensator plate
Observer
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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
  earths motion. The results disproved the
  existence of the ether, which contradicted the
  Newtonian physics but was explained by Einsteins
  special relativity (1905).

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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.

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• 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.

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• The condition for a dark fringe in Fraunhofer
  single slit diffraction with n1,2,3
• 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 m0,1,2,

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• 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 d1/6000001.67x10-
  6m.
• The angular deviation of the violet is

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• The angular deviation of the red is
• Hence, the first order visible spectrum includes
  an angle of 24.3-13.910.9 degree.

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Diffraction of x-rays by a crystal

• X-rays were discovered by Roentgen in 1895. Its
  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.

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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.

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Rosalind Franklin
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• 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.

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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.

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• 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.

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• 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.

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• 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.

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Franklin's Photo 51
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• She was able to get this remarkable imagethe
  clearest image of DNA ever created up until that
  timewith 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.

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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.

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• 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.

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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.

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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.

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• 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.

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• 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

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• 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?

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• 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.

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• 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?

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• 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.