# Light http://jsu.ac.ir/~dvdtlb/.

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Light

Briefly In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. Primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization. Its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. The study of light, and the interaction of light and matter is termed optics, is an important research area in modern physics.

Light, which is emitted and absorbed in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality.

Ray Theory – Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength (multimode fibers)

Archimedes of Syracuse (Greek: Ἀρχιμήδης)
Dossier of History Archimedes of Syracuse (Greek: Ἀρχιμήδης) Born c. 287 BC Syracuse, Sicily; Magna Graecia Died c. 212 BC (aged around 75) Syracuse Residence Syracuse, Sicily Fields Mathematics, Physics, Engineering, Astronomy, Invention Known for Archimedes' Principle, Archimedes' screw, Hydrostatics, Levers, Infinitesimals

Ptolemy (Batlamyus) Born c. AD 90; Egypt Died c. AD 168 (aged 77–78) Alexandria, Egypt Occupation mathematician, geographer, astronomer, astrologer

Law of reflection the angle of incidence equals angle of reflection
angles are measured from normal

Refraction Snell’s Law
index of refraction, n, where c = speed of light in a vacuum and v = speed of light in that medium nair = 1 nglass = 1.5 Snell’s Law

Total Internal Reflection
Incident angle where refracted angle (2) is 90 is the critical angle at incident angles greater than critical angle, light is totally internally reflected Geometric Optics

Guided Propagation Along the Optical Fiber
The structure of a typical single-mode fiber. 1. Core: 8 µm diameter 2. Cladding: 125 µm dia. 3. Buffer: 250 µm dia. 4. Jacket: 400 µm dia.

Plato’s fire Full name Plato (Πλάτων) Born c. 428–427 BC; Athens Died c. 348–347 BC (age approx 80); Athens Era Ancient philosophy Region Western Philosophy School Platonism Influenced by: Socrates, Homer, Hesiod, Aesop, Protagoras, Parmenides, Pythagoras, Orphism, … Influenced: Most of subsequent western philosophy, including Aristotle, Augustine, Neoplatonism, Cicero, Plutarch, Stoicism, Anselm, Machiavelli, Descartes, Hobbes, Leibniz, Mill, Schopenhauer, Nietzsche, Heidegger, Arendt, Gadamer, Imam Khomeini, Russell and countless other philosophers and theologians. So much of fire as would not burn, but gave gentle light, they formed into a substance related to the light of everyday life, and the pure fire which is within us and related to it they made to flow through the eyes in a stream smooth and dense, compressing the whole eye and specially the center part, so that it kept out everything of a coarser nature and allowed to pass only its pure element.

Alhazen (Ibn al-Haytham)
End of visual rays Alhazen (Ibn al-Haytham) Born July 1, 965 CE (354 AH) Basra in present-day Iraq, Buyid Persia Died March 6, 1040 (aged 74) (430 AH) Cairo, Egypt, Fatimid Caliphate Fields physicist and Mathematician Known for Book of Optics, Doubts Concerning Ptolemy, On the Configuration of the World, The Model of the Motions, Treatise on Light, Treatise on Place, scientific method, experimental science, experimental physics, experimental psychology, visual perception, analytic geometry, non-Ptolemaic astronomy, celestial mechanics Influences Aristotle, Euclid, Ptolemy Influenced Averroes, Witelo, Roger Bacon, Kepler Over-confident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile. “There is no vision unless something comes from the visible object to the eye, whether or not anything goes out” camera obscura

Measurements of the Speed of Light

By watching the moons circling Jupiter through a telescope, he was able to observe the exact time that the moon moved behind Jupiter. After many years of observations, however, he noticed that the time intervals between eclipses were not always the same. (position 1): The light from the eclipsing moon sometimes took a relatively short time to reach earth. (position 2): when earth was in a farther part of its orbit, the light from Jupiter's moon would take longer to reach earth. Roemer was able to use the time difference and the diameter of Earth's orbit to calculate the speed of light. He determined it to be about 185,000 miles per second, or 296,000 kilometers per second, very close to the actual value. Ole Rømer Born 25 Sept. 1644; Århus Died 19 Sept (aged 65) Copenhagen Nationality Danish

Light Speed measurement
Hippolyte Fizeau Born Sept. 23, 1819 Paris Died Sept. 18, 1896 Venteuil Nationality French Fields Physics Known for Doppler Effect Fizeau-Foucault apparatus Capacitor using a pair of toothed wheels driven by a very fast motor. As the toothed wheels turned, a beam of light shone through the gaps between the teeth of one wheel would on ly pass through the gaps in the other wheel if they lined up. Knowing the speed that the wheels were turning allowed him to calculate the speed of light with an accuracy similar to Roemer's.

Albert Abraham Michelson
Michelson's method was to shine a light on the rotating hexagonal mirror, which then reflected to the other mirror 35 kilometres away. The mirror needed to make one-eighth of a rotation in the time it took the light to make the return trip. This meant that the octagonal mirror had to be turning at about rpm. From the round trip distance the light travelled, and the period of rotation of the octagonal mirror, the speed of light was determined quite accurately. Albert Abraham Michelson Born December 19, 1852; Strzelno, Kingdom of Prussia Died May 9, 1931; Pasadena, California United States; Physics Alma mater: United States Naval Academy, University of Berlin; Doctoral advisor: Hermann Helmholtz; Doctoral students: Robert Millikan Known  For Speed of light Michelson-Morley experiment Notable awards Nobel Prize for Physics (1907) Copley Medal (1907) Henry Draper Medal (1916)

The Search for Aether Michelson - Morley Experiment(1887)
No aether drift was detected, despite repeated experiments! Conclusion: jundi Shapur University, Dezful is the center of the Universe or There is No Aether

Measuring Earth’s Movement through the Aether
Rotating the apparatus would give the direction of the aether drift Showed that there is no such thing as aether (nor any need for it). Light is perfectly happy traveling in a vacuum. The speed of light is the same in any direction, which explains the null result of Michelson and Morley.

Wave Theory – Light travels as a transverse electromagnetic wave

Magnet and Magnetism certain iron oxides were discovered in various parts of the world, notably in Magnesia in Asia Minor, that had the property of attracting small pieces of iron. Magnesia within Greece

discovered that electric currents create magnetic fields.
Hans Christian Ørsted Born 14 August 1777 Rudkøbing, Denmark Died 9 March 1851 (aged 73)Copenhagen, Denmark Nationality Danish Fields physics, chemistry Known for electromagnetism discovered that electric currents create magnetic fields.

relates the integrated magnetic field around a closed loop to the electric current passing through the loop. André-Marie Ampère Born 20 January 1775 Parish of St. Nizier, Lyon, France Died 10 June 1836 (aged 61) Marseille, France Residence France Nationality French Fields Physics Institutions Bourg-en-Bresse École Polytechnique Known for Ampere's Law

Heinrich Rudolf Hertz Born February 22, 1857 Hamburg, Germany Died January 1, 1894 (aged 36) Bonn, Germany Fields Physics; Electronic Engineering Institutions University of Kiel University of Karlsruhe University of Bonn Doctoral advisor Hermann von Helmholtz Known for Electromagnetic radiation Photoelectric effect

NATURE OF WAVES Waves (Def.) – A wave is a disturbance that transfers energy. Medium – Substance or region through which a wave is transmitted. Speed of Waves – Depends on the properties of the medium.

Transverse Waves Energy is perpendicular to direction of motion
Moving photon creates electric & magnetic field Light has BOTH Electric & Magnetic fields at right angles!

Light consists of a varying electric and magnetic field
So, What is Light? Light consists of a varying electric and magnetic field

Electromagnetic Spectrum
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. A photon of ultra-violet radiation carries more energy than a photon of infrared radiation. © 2000 Microsoft Clip Gallery

Invisible Spectrum Radio Waves Short Wavelength Microwave
Def. – Longest wavelength & lowest frequency. Uses – Radio & T.V. broadcasting. Short Wavelength Microwave Infrared Rays Def – Light rays with longer wavelength than red light. Uses: Cooking, Medicine, T.V. remote controls © 2000 Microsoft Clip Gallery

The shortest wavelengths in the visible spectrum are purple, and the longest wavelengths are red.
Visible light has wavelength in a range from about 380 nanometers to about 740 nm, with a frequency range of about 405 THz to 790 THz.

Electromagnetic Spectrum
Visible Spectrum – Light we can see Roy G. Biv – Acronym for Red, Orange, Yellow, Green, Blue, Indigo, & Violet. Largest to Smallest Wavelength.

Visible Light We now know what we see is part of the electromagnetic spectrum. We know that the light waves enter our eye, and stimulate parts of it that cause a electrical impulse to be sent to the brain which creates this visual image. But everything does not emit radiation. How do we see those things? And why cant we see a window?

Only red light is reflected
Seeing colour The colour an object appears depends on the colours of light it reflects. For example, a red book only reflects red light: Homework White light Only red light is reflected

A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colours: White light

Using coloured light If we look at a coloured object in coloured light we see something different. For example, consider a football kit: Shirt looks red White light Shorts look blue

In different colours of light this kit would look different:
Red light Shirt looks red Shorts look black Shirt looks black Blue light Shorts look blue

Seeing things We know that when waves run into a boundary they are partially transmitted and partially reflected. Light behaves as a wave, so it to is reflected. Therefore, an object does not need to emit photons itself to be seen, it just has to reflect light back to our eyes where we can detect it. Objects that do not allow light to pass through them are called opaque. Objects that allow light to pass through them are considered transparent. Objects in between are called translucent. © 2003 Mike Maloney

Invisible spectrum (cont.).
Ultraviolet rays. Def. – EM waves with frequencies slightly higher than visible light Uses: food processing & hospitals to kill germs’ cells Helps your body use vitamin D. X-Rays Def. - EM waves that are shorter than UV rays. Uses: Medicine – Bones absorb x-rays; soft tissue does not. Lead absorbs X-rays.

Light sources Thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue colour as the peak moves out of the visible part of the spectrum and into the ultraviolet.

Atoms: emit and absorb light at characteristic energies
Atoms: emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.

Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence.
In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.

Certain other mechanisms can produce light:
Scintillation; electroluminescence; sonoluminescence; triboluminescence; Cherenkov radiation; Bioluminescence. When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include: Radioactive decay; Particle–antiparticle annihilation

Kinds of Spectra

Polarization Polarization is a phenomenon of light that is used in sun-glasses and 3-D movies. Play with the two polarizing filters for a few minutes and note what is happening and see if you can think of any reasons for it. © 2003 Mike Maloney

Polarization Hint Light vibrates in all directions.
A polarizing filter acts like a picket fence. It only lets certain direction vibrations pass through it. Therefore, if you pass light through two of them you can completely block the light from passing through. HOW? © 2003 Mike Maloney

Polarization http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=692.0

Light Energy When EM radiation interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries. Atoms As atoms absorb energy, electrons jump out to a higher energy level. Electrons release light when falling down to the lower energy level. Photons - bundles/packets of energy released when the electrons fall. Light: Stream of Photons

Quantum Theory – Light consists of small particles (photons)
Newton’s Corpuscular Theory of Light - light consists of small particles, because it: • travels in straight lines at great speeds • is reflected from mirrors in a predictable way Position x Momentum p = mv

Why the Photon is Necessary
Electron transitions in the Bohr model of the atom and the subsequent emission of light provides an example of when light should be viewed as a photon. There are two further pieces of evidence of this particle-like nature of light: • photon scattering • photoelectric effect

Scattering One experiment which provides conclusive proof of a particle nature of objects is to scatter two objects off of each other, as in the collision of two billiard balls. This experiment with light and small atoms has been done, and is called Compton scattering. The results of this experiment are completely at odds with predictions made if light is viewed only as a wave. Measurements show that the frequency of the scattered wave is changed, which does not come out of a wave picture of light. However, when the light is viewed as a photon with energy proportional to the associated light wave, excellent agreement with experiment is found.

Photoelectric Effect Another compelling proof for the photon nature of light is the photoelectric effect. In this effect, light is shone at a metal plate and it is found that electrons are ejected. These electrons then get accelerated to a nearby plate by an external potential difference, and a photoelectric current is established. This effect, which arises in devices such as automatic door openers, burglar alarms, light detectors, and photocopiers, cannot be explained using a wave picture of light.

Einstein’s Photoelectric Effect
Only light with a frequency greater than a certain threshold will produce a current Current begins almost instantaneously, even for light of very low intensity Current is proportional to the intensity of the incident light

Planck’s Quantum Postulate
Energy of radiation can only be emitted in discrete packets or quanta, i.e., in multiples of the minimum energy E = hf where h is a new fundamental constant of nature: h = 6.63 x Joules sec

We Believe in Photons Red light is used in photographic darkrooms because it is not energetic enough to break the halogen-silver bond in black and white films Ultraviolet light causes sunburn but visible light does not because UV photons are more energetic Our eyes detect color because photons of different energies trigger different chemical reactions in retina cells

Special Relativity: Postulates
Principle of Relativity All the laws of physics are the same in all inertial reference frames, i.e. frames moving at constant velocities w/respect to each other. Constant Speed of Light The speed of light, c = 3×108 m/s, is equal in all inertial frames, regardless of the velocity of the observer or the light source. Consequences of Special relativity “Slowing” down of clocks (time dilation) and length contraction in moving reference frames as measured by an observer in another reference frame.

Galilean Transform: Predicts Preferred Ref. Frame
According to the Galilean transform, the travel time t1 across & back a river is shorter than the travel time t2 up & down a river. Light moving in an “ether” is an analogous problem. Can light have different travel times? t1 t2 Phys Baski Relativity I

Galilean Transform: NO Preferred Ref. Frame for Light!
In 1800’s, scientists thought that light propagated through some type of “ether.” Michelson-Morley Experiment (1887) Test if ether exists and sets “preferred” reference frame. Analogous to rowboat in river. Measure light speed relative to earth’s motion (// and ) using an interferometer (fringes). Result: No detection of “ether” No detectable shift in interference fringes occurred, indicating that light speed DID NOT depend on direction. Phys Baski Relativity I

Doppler Shift Doppler shift causes change in measured frequency.
When a light source moves towards an observer, the light frequency is shifted higher (i.e. blue shift). When a light source moves away from an observer, the light frequency is shifted lower (i.e. red shift). Only difference with “classical” Doppler shift for sound is the incorporation of time dilation (causes square root factor). Approaching - blue shift Note: For a receding source, switch signs. Phys Baski Relativity I

Cool Thing About Light It can be thought of as both a particle and a wave, so called “particle-wave duality” Lower energy (longer wavelength) light acts predominately like a wave High energy (shorter wavelength) light acts predominately like a particle

Cool Things Light Can Tell Us
It can tell us what you are made out of It can tell us if you are moving toward or away from us It can tell us how far away you are or (if we already know that) how energetic you are It can tell us your temperature

Another Way to Look at a Spectrum

Spectral Lines Lines from excited sodium gas in the laboratory