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Waves

Waves. (part 3). Optics: Chapter 15. Nature of Light. History. 17 th century Newton-light consisted of particles Huygens-light consisted of waves They both believed that a medium was necessary for propagation of light

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Waves

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  1. Waves (part 3)

  2. Optics: Chapter 15 Nature of Light

  3. History • 17th century • Newton-light consisted of particles • Huygens-light consisted of waves • They both believed that a medium was necessary for propagation of light • Maxwell (1864)- a sort of symmetry between electric and magnetic fields

  4. Maxwell continued • Electric and magnetic fields act together to produce an electromagnetic wave that travels with the speed of light. • Visible light is electromagnetic • Used to be thought of separately from electricity and magnetism • Mathematical form

  5. Hertz • Experimental evidence • Further investigation showed that electromagnetic waves exhibit properties of reflection, refraction, interference, diffraction, and polarization

  6. Production of EM waves • Produced because of accelerated motion of electric charges • Electron accelerateproduce changing electric fieldsgenerate changing magnetic fields in a plane perpendicular to the electric field plane

  7. Production of EM waves • As this continues, a self-propagating EM wave is produced • Charge oscillates with simple harmonic motion • Sine curves are produced perpendicular to each other and to the direction of the wave velocity • Waves are periodic and transverse

  8. Production of EM waves

  9. Field directions

  10. Right-hand rule

  11. Which direction is the magnetic field?

  12. Speed of light • All EM waves travel at the same speed in a vacuum • Free space velocity of light (c) • Velocity of 2.998 x 108 m s-1

  13. EM waves • Can also propagate in matter • When they travel in different media, such as glass, they are refracted and therefore travel at a different speed • Carries momentum and energy

  14. EM waves • As wavelength gets longer, the frequency decreases • There is a range of wavelength values that EM waves can have • Entire range is called the electromagnetic spectrum • Range of wavelengths from about 108 m to 10-17 m (frequency 1 Hz to 1025 Hz) have been studied by scientists.

  15. EM waves • Can be produced and detected in different ways, all waves in the EM spectrum behave as predicted by Maxwell’s theory • They have energy or electromagnetic radiation • Can only be emitted if energy is supplied to the source of radiation • Ex: heating of food in a microwave

  16. EM waves • Quantum physics-1900- Max Planck • Radiation absorbed or emitted by a black body could not be explained using classical physics • EM radiation could not be emitted or absorbed continuously • It is emitted of absorbed in “little bursts” or “packets” of energy (photons) • E = hf • E is the energy of the photon in J • h is called Plank’s constant (6.6x10-34 J s)

  17. EM waves and the line of sight

  18. Electromagnetic spectrum • Representation of the different radiation bands • Certain bands given special names

  19. Electromagnetic Spectrum

  20. Radio Waves • Lowest frequency • 30 Hz to greater than 3000 MHz • Radio communication • AM and FM • Television • CB radio • Scanning devices in MRI • Government regulates bandwidth in order to avoid congestion of the airwaves

  21. Radio waves • Generated by an electric current called an oscillator and are radiated from an aerial • A tuned oscillatory electric current detects the radio waves • Easily reflected off surfaces, making them ideal for communication technology

  22. Receiving radio waves

  23. Microwaves • Mobile phone, satellite communication, radar, and cooking • Main carriers of communication between repeater stations • Produced by special electronic semi-conductor devices called Gunn diodes, or by vacuum tube devices • Radar-short pulses of microwaves

  24. Infra-red Radiation • Wavelength slightly longer than the red end of the visible spectrum • Receive warmth from the Sun and other heat sources • Detected by: skin, thermometers, thermistors, photoconductive cells, special photographic film

  25. Visible light • Wavelength of 400 nm to 700 nm • Detected by stimulating nerve endings of the retina of the eye • Eye is most sensitive to the green and yellow parts of the spectrum • Generated by re-arrangement of outer orbital electrons in atoms and molecules • Excited electrons emit light and other EM radiation when they lose energy

  26. Ultra-Violet radiation • Waves between about 10.7 m to 10.9 m • Generated by the orbital electrons of atoms of the Sun, and other instruments • Can cause photochemical reactions • Has the ability to ionize atoms • This is why ionosphere is produced in the atmosphere

  27. X-radiation • Waves 10-8 m to 10-18 m • Generated by rapid stopping of deflection of fast-moving electrons when they strike a metal target or other hard object • Generated by sudden change in energy of innermost orbital electrons in atoms

  28. X-rays • Rays are weakly absorbed by the skin and soft tissues • Pass through bodies freely, except when they hit dense material • “hard” x-rays are near the gamma end • “soft” x-rays are near the UV end

  29. Gamma radiation • Overlaps with the X-ray region • Use in cancer therapy overlaps with X-ray use in radiotherapy • High frequency and highly penetrating • Produced by natural and artificial radioactive materials

  30. Experimental determination of the speed of light • Galileo-flash from an artillery gun was seen before the sound was heard • Experiment-person with a lantern stood on one hill and an observer stood on another hill • Observer timed how long the flash took to reach him

  31. Experimental determination of the speed of light • Romer (1676)- 1st recognized prediction • Observed Jupiter’s moon-noticed that the period between eclipses is 42.5 hours • There was an irregularity in times between successive eclipse periods as the Earth orbits the Sun

  32. Experimental determination of the speed of light • After some reasoning, he estimated that the extra time for the light to reach Earth was 22 minutes • Huygens used Romer’s value and came up with a value 2/3 the presently accepted value • Fizeau made the 1st non-astronomical measurement (1849)

  33. Fizeau’s experiment

  34. Fizeau’s results

  35. Experimental determination of the speed of light • In 1860, Foucalt improved Fizeau’s method • From 1880-1930, Michelson made a series of precise measurements to determine the speed of light

  36. Dispersion

  37. Prisms • Narrow beam of white light undergoes refraction on entering a prism • Light spreads out into a spectrum of colors • Red at one side, purple at the other • Spectrum of white light is continuous because color bands gradually change from one color to the next without gaps.

  38. Prismatics

  39. Dispersion • Separation of white light into its component colors • First explained by Newton • Isolated a particular spectrum color produced when white light was passed through a prism • Then passed through a second prism • Found that there was no color change • Showed that the colors could be recombined or synthesized to produce white light

  40. Dispersion • Refractive index of the prism material is different for each color • Greater for violet than red • Red light has a longer wavelength than orange light, yellow light, and so on • Each wavelength has a different speed, and different media have different refractive indexes

  41. Dispersion • The separation of a mixture of different wavelengths initially traveling together, when they enter a new medium where velocity depends on a frequency • Each frequency has a different refractive index • Example: refractive index for glass is smaller for red light than it is for blue light

  42. Prism spectrometer • Examines wavelengths of visible light p702 • 3 main parts • The collimator-a system that produces a parallel beam of light • The prism • A typical refracting telescope that can be moved through an angle

  43. Dispersion in a raindrop

  44. How rainbows are produced

  45. I = T + A+ S • I – incoming • T – transmitted • A – absorbed • S - scattered

  46. EM Waves • Have energy (EM Radiation) • Only emitted if E supplied to source (charge that is accelerated) • Energy may be absorbed &.. • Detected (heat food in microwave) • Re-radiated & transmit • Collisions w/ particles = scattering • Scattered rad may have diff freq, directions, polariz • May interact @ atomic or molecular level

  47. Light from the Sun • Absorbed in higher atm? • Rayleigh Scattering? • Colors of sky? • What scatters in the atm? • Greenhouse gasses?

  48. Lasers • Light Amplification by Stimulated Emission of Radiation • Has a power source and a light-amplifying substance • Power source provides the energy that causes atoms in the light-amplifying substance to become excited • Common laser- helium-neon gas mixture

  49. Helium-neon laser

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