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Light Briefly In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or.

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Presentation on theme: "Light Briefly In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or."— Presentation transcript:


2 Light

3 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.

4 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.

5 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)

6 Dossier of History


8 7 Law of reflection – the angle of incidence equals angle of reflection – angles are measured from normal

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

10 Geometric Optics 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

11 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. Guided Propagation Along the Optical Fiber

12 Plato’s fire 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.

13 End of visual rays “There is no vision unless something comes from the visible object to the eye, whether or not anything goes out” camera obscura Over-confident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile.Nile

14 Measurements of the Speed of Light


16 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.

17 Light Speed measurement 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.

18 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.

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

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

21 Wave Theory – Light travels as a transverse electromagnetic wave

22 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

23 discovered that electric currents create magnetic fields.

24 relates the integrated magnetic field around a closed loop to the electric current passing through the loop.

25 The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.electromotive forcemagnetic flux



28 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.

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

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

31 Electromagnetic Spectrum © 2000 Microsoft Clip Gallery 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.

32 Invisible Spectrum Radio Waves 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

33 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.




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

38 37 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?

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

40 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 White light

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

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

43 © 2003 Mike Maloney42 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.

44 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.

45 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.

46 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.

47 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.

48 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.fireflies Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence.fluorescence Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.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. Cathodoluminescencecathode ray tubetelevision setscomputer monitors

49 Certain other mechanisms can produce light: Scintillation; electroluminescence; sonoluminescence; triboluminescence; Cherenkov radiation; Bioluminescence. Scintillationelectroluminescencesonoluminescence triboluminescenceCherenkov 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 annihilationRadioactive decay antiparticle

50 Kinds of Spectra

51 © 2003 Mike Maloney50 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.

52 © 2003 Mike Maloney51 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?

53 © 2003 Mike Maloney52 Polarization

54 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

55 Quantum Theory – Light consists of small particles (photons) u 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

56 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

57 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.

58 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.

59 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

60 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

61 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

62 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 = 3×10 8 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.

63 Phys BaskiRelativity I Galilean Transform: Predicts Preferred Ref. Frame According to the Galilean transform, the travel time t 1 across & back a river is shorter than the travel time t 2 up & down a river. Light moving in an “ether” is an analogous problem. – Can light have different travel times? t1t1 t2t2

64 Phys BaskiRelativity 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.

65 Phys BaskiRelativity 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.

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

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

68 Another Way to Look at a Spectrum

69 Spectral Lines Lines from excited sodium gas in the laboratory


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