Wave Properties Of Light

1. Wave Properties Of Light Chapter 18

2. Electromagnetic Waveforms • The and fields are perpendicular to each other • Both fields are perpendicular to the direction of motion • Therefore, electromagnetic waves are transverse waves • With all periodic waves • Since v = c in a vacuum [11.1]

3. Electromagnetic Waves, Summary • A static electric charge produces an electric field. • A uniformly changing (moving) electric field produces an magnetic field • A uniformly changing (moving) magnetic field produces a electric field **But NONE of these produces an EM WAVE. For this you need an accelerating charge.**

4. Velocity of Light c = 3 x 108m/s (In a vacuum) Slower values in other mediums, even air slows down light, but frequency will stay the same

5. Frequency, Wavelength and Velocity • Wavelength alters along with velocity in order to keep frequency constant • Objects’ COLOR is determined by frequency, NOT wavelength • Wavelengths normally listed in the EM spectrum for the visual range are VACUUM wavelengths (in space) • As light passes from that vacuum of space into a medium with a higher index of refraction, its velocity is reduced, and thus wavelength must also reduce in order to preserve frequency and color

6. Electromagnetic-Photon Spectrum

7. The Spectrum of EM Waves • Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths • c = ƒλ • Wavelengths for visible light range from 400 nm to 700 nm • There is no sharp division between one kind of em wave and the next

8. The EM Spectrum • Note the overlap between types of waves • Visible light is a small portion of the spectrum • Types are distinguished by frequency or wavelength

9. Radiowaves • Radio Waves • Used in radio and television communication systems • Wavelengths range from 100s of meters to less than a cm

10. 0 Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc.

11. 0 Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light Use interferometry to improve resolution!

12. Microwaves • Microwaves • Wavelengths from about 1 mm to 30 cm • Well suited for radar systems • Microwave ovens are an application

13. Infrared • Infrared waves • Incorrectly called “heat waves” • 1mm down to 700nm • Produced by hot objects and molecules • Readily absorbed by most materials

14. Visible Light • Visible light • Part of the spectrum detected by the human eye • Most sensitive at about 560 nm (yellow-green)

15. Ultraviolet • Ultraviolet light • Covers about 400 nm to 0.6 nm • Sun is an important source of UV light • Most UV light from the sun is absorbed in the stratosphere by ozone

16. X-rays • X-rays • Most common source is acceleration of high-energy electrons striking a metal target • Used as a diagnostic tool in medicine

17. Gamma Rays • Gamma rays • Emitted by radioactive nuclei • Highly penetrating and cause serious damage when absorbed by living tissue

18. Interference, Diffraction and Polarization 18.2

19. Proving Light Is A Wave Thomas Young, in 1807, used Diffraction to prove that light experiences both Constructive and Destructive Interference.

20. Diffraction • The shape of a wave front is altered (bent) as it passes through a hole or slit in another medium. • A Diffraction Grating is a screen with a series of slits which create constructive/destructive interference patterns • A Spectrometer uses a diffraction grating to create a spectrum measuring the wavelengths on incoming light. This can be used to identify materials emitting that light

21. Diffraction

22. Polarization • Light travels as a transverse wave, and it can thus displace within a plane with a specific orientation. • White Light consists of all wavelengths essentially travelling in all polarizations – it is UNpolarized. • Polarization of a light wave is defined by the orientation of the Electric Wave

23. Polarizers • Filter out all light wave polarizations EXCEPT one • The light transmitted through a polarizer is now “polarized”—it has a single polarization • Can be used to block out some portion of sunlight (sunglasses), create images on an LCD screen by filtering certain values of RGB light and even to identify mineral composition of rocks viewed in thin section

25. Relativity 18.3

26. Foundation of Special Relativity • Reconciling of the measurements of two observers moving relative to each other • Normally observers measure different speeds for an object • Special relativity relates two such measurements **Rests on the foundation that the speed of light (c) is the same for ALL observers, regardless of either THEIR motion or any motion of the light SOURCE.**

27. Consequences of SR • Time Dilation • Length Contraction • Increased Mass of particles • Loss of Simultaneity **But FIRST---a little historical perspective….**

28. Galilean Relativity • Choose a frame of reference • Necessary to describe a physical event • According to Galilean Relativity, the laws of mechanics are the same in all inertial frames of reference • An inertial frame of reference is one in which Newton’s Laws are valid • Objects subjected to no forces will move in straight lines

29. Galilean Relativity – Example • A passenger in an airplane throws a ball straight up • It appears to move in a vertical path • This is the same motion as when the ball is thrown while at rest on the Earth • The law of gravity and equations of motion under uniform acceleration are obeyed

30. Galilean Relativity – Example • There is a stationary observer on the ground • Views the path of the ball thrown to be a parabola • The ball has a velocity to the right equal to the velocity of the plane

31. Galilean Relativity – Example • The two observers disagree on the shape of the ball’s path • Both agree that the motion obeys the law of gravity and Newton’s laws of motion • Both agree on how long the ball was in the air • Conclusion: There is no preferred frame of reference for describing the laws of mechanics

32. Galilean Relativity – Limitations • Galilean Relativity does not apply to experiments in electricity, magnetism, optics, and other areas • Results do not agree with experiments • The observer should measure the speed of the pulse as v+c • Actually measures the speed as c

33. Luminiferous Ether • 19th Century physicists compared electromagnetic waves to mechanical waves • Mechanical waves need a medium to support the disturbance • The luminiferous etherwas proposed as the medium required (and present) for light waves to propagate • Present everywhere, even in empty space • Massless, but rigid medium • Could have no effect on the motion of planets or other objects

34. Verifying the Luminiferous Ether • Associated with an ether was an absolute framewhere the laws of e & m take on their simplest form • Since the earth moves through the ether, there should be an “ether wind” blowing • If v is the speed of the ether relative to the earth, the speed of light should have minimum (b) or maximum (a) value depending on its orientation to the “wind”

35. Michelson-Morley Experiment • First performed in 1881 by Michelson • Repeated under various conditions by Michelson and Morley • Designed to detect small changes in the speed of light • By determining the velocity of the earth relative to the ether

36. Michelson-Morley Equipment • Used the Michelson Interferometer • Arm 2 is aligned along the direction of the earth’s motion through space • The interference pattern was observed while the interferometer was rotated through 90° • The effect should have been to show small, but measurable, shifts in the fringe pattern

37. Michelson-Morley Results • Measurements failed to show any change in the fringe pattern • No fringe shift of the magnitude required was ever observed • Light is now understood to be an electromagnetic wave, which requires no medium for its propagation • The idea of an ether was discarded • The laws of electricity and magnetism are the same in all inertial frames • The addition laws for velocities were incorrect

38. Lorentz-FitzGerald Contraction • Proposed in 1892 by G. Fitzgerald to retain the aether wind. • Every object moving at speed v contracts along the direction of motion by a factor equal to Where b = v/c • Re-emerged over a decade later as part of Einstein’s Special Theory of Relativity

39. Albert Einstein • 1879 – 1955 • 1905 published four papers • 2 on special relativity • 1916 published about General Relativity • Searched for a unified theory • Never found one

40. Einstein’s Principle of Relativity • Resolves the contradiction between Galilean relativity and the fact that the speed of light is the same for all observers • Postulates • The Principle of Relativity: All the laws of physics are the same within all inertial frames • The Constancy of the Speed of Light: the speed of light in a vacuum has the same value in all inertial reference frames, regardless of the velocity of the observer or the velocity of the source emitting the light

41. The Principle of Relativity • This is a sweeping generalization of the principle of Galilean relativity, which refers only to the laws of mechanics • The results of any kind of experiment performed in a laboratory at rest must be the same as when performed in a laboratory moving at a constant speed past the first one • No preferred inertial reference frame exists!!! • It is impossible to detect absolute motion!!!

42. Consequences of Special Relativity • Restricting the discussion to concepts of length, time, and simultaneity • In relativistic mechanics • There is no such thing as absolute length • There is no such thing as absolute time • Events at different locations that are observed to occur simultaneously in one frame are not observed to be simultaneous in another frame moving uniformly past the first

43. Time Dilation • The vehicle is moving to the right with speed v • A mirror is fixed to the ceiling of the vehicle • An observer, O’, at rest in this system holds a laser a distance d below the mirror • The laser emits a pulse of light directed at the mirror (event 1) and the pulse arrives back after being reflected (event 2)

44. Time Dilation, Moving Observer • Observer O’ carries a clock • She uses it to measure the time between the events (ΔtS) • tSis the proper time (see forward 5 slides for proper time) • She observes the events to occur at the same place • ΔtS = distance/speed = (2d)/c

45. Time Dilation, Stationary Observer • Observer O is a stationary observer on the earth • He observes the mirror and O’ to move with speed v • By the time the light from the laser reaches the mirror, the mirror has moved to the right • The light must travel farther with respect to O than with respect to O’

46. Time Dilation, Observations • Both observers must measure the speed of the light to be c • The light travels farther for O • The time interval, ΔtM, for O is longer than the time interval for O', ΔtS

47. d Time Dilation, Time Comparisons where • Observer O measures a longer time interval than observer O'

48. Time Dilation, Summary • The time interval Δt between two events measured by an observer moving with respect to a clock is longer than the time interval ΔtS between the same two events measured by an observer at rest with respect to the clock • A clock moving past an observer at speed v runs more slowly than an identical clock at rest with respect to the observer by a factor of -1

49. Identifying Proper Time • The time interval ΔtS is called the proper time • The proper time is the time interval between events as measured by an observer who sees the events occur at the same position • You must be able to correctly identify the observer who measures the proper time interval

50. Alternate Views • The view of O’ that O is really the one moving with speed v to the left and O’s clock is running more slowly is just as valid as O’s view that O’ was moving • The principle of relativity requires that the views of the two observers in uniform relative motion must be equally valid and capable of being checked experimentally