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Light

Light. Exploring what light is, color, frequencies and shifts. History of Light. Ancient Greeks thought that every object emitted tiny particles just like that object that your eye picked up. Socrates and Plato.

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Light

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  1. Light Exploring what light is, color, frequencies and shifts

  2. History of Light • Ancient Greeks thought that every object emitted tiny particles just like that object that your eye picked up

  3. Socrates and Plato • Both believed that your eye had streamers or filaments that were invisible and left your eye and attached to everything that you saw

  4. Michelson-Morley Experiment Done multiple times to determine the properties of ether

  5. Speed of Light The speed of light is a constant 299,792,458 rounded to 3.0 X 10^8 m/s Light always travels at this speed. (except when it doesn’t)

  6. Teacher Example If the Sun exploded, how long would we have to wait until we knew about it if the distance from the Sun to the earth was 1.5 X 1011m?

  7. Student Example #1 – 60 s Voyager 2 is now 1 X 10^12 m away from the Earth. How long will it take for the signal to reach Earth? Round to the whole number

  8. Teacher Example In 1936 the first television signal left Earth. If an alien culture sends us an answer this year. Then how far away is the alien culture assuming they replied right away.

  9. Student Example #2 - 90 s A supernova occurs from a star that is known to be 2.5 X 10^16 m away. The light reaches the Earth this year, what year did the supernova occur?

  10. From the 1800-1900’s • Young's Double Slit Experiment

  11. Photoelectric Effect Photoelectric Effect

  12. Student Example #3 – 30 s Which of the experiments gave evidence for the wave-like nature of light? • Michelson-Morley experiment • Young’s Double Slit • Photoelectric Effect • Two or more • All of the above

  13. Student Example #4 – 30 s Which of the experiments gave evidence that light can travel in a vacuum? • Michelson-Morley experiment • Young’s Double Slit • Photoelectric Effect • Two or more • All of the above

  14. Photons • Are released by atoms as they lose energy in discrete amounts or quanta

  15. Energy Levels Light is created by electrons falling from orbit

  16. Photons of Light • Photons have discrete amounts of energy at multiples of Planck’s Constant, which has a value of 6.626068 × 10-34 m2 kg/s • If energy is lost or absorbed by an atom in the form of light the frequency of f of the light is related by E = h f

  17. Teacher Example Determine the frequency of a mole of photons that posses a total energy of 1 mega-joule

  18. Student Example #5 – 60 s Determine the energy of a single photon containing violet light which has a frequency 7.5 X 1014 Hz

  19. Wave-Particle Duality • Light just like all other matter acts as both a wave and a particle • Light as wave and Photon

  20. Electromagnetic Waves • Radio • Light • U/V • X – rays • Oscillate in both electric and magnetic fields

  21. Electromagnetic Spectrum

  22. Radio Waves • Low frequency • Low energy • High wave • Carry information to radios

  23. Microwaves • Same rays used in cellular phones as used to cook food

  24. Infrared Waves • Measure of Heat

  25. Visible Light ROYGBIV Red light has less energy than violet

  26. Why are New Fire Trucks Yellow-Green?

  27. Green is Easier to See • Which one is more in the middle of human vision? • Humans are genetically pre-disposed to see green

  28. Ultraviolet • Causes sunburn • Special images of stars using u/v rays • Much higher in frequency and energy

  29. X-rays Very High Energy that penetrates skin and produces images Dangerous Known to cause cancer with repeated exposure

  30. Gamma Rays • One Gamma Ray destroys metal completely • Very Dangerous • One exposure could cause death

  31. Teacher Example • Which color of light has the greatest intensity?

  32. Teacher Example Which of the following penetrate the skin? • Radio • Microwave • Infrared • Visible light • Ultraviolet

  33. Clicker Quiz

  34. Student Quiz #1 – 30 s Which of the following is the least harmful to humans? • Radio • Microwave • Infrared • Visible light • Ultraviolet

  35. Student Quiz #2 – 45 s Which Jedi has an inherent advantage in a light saber duel? • Mace Windub) Luke Skywalker c) Obi Wan Kenobi d) Darth Vader e) Qui-Gon Jinn

  36. Student Example #3 – 30 s What are Gamma rays good for? • Secure communication • Getting super powers • Treating cancer • Glowing in the dark

  37. Student Quiz #4 What is the frequency of a single photon with an energy of 6.626 X 10^-32 J? Round to the nearest whole number

  38. Student Quiz #5 100 moles of photons have a total energy of 0.00000015 J. What is the frequency of the light assuming all photons are the same? Round to the nearest photon

  39. Electromagnetic Waves c = λf

  40. Teacher Examples Determine the wavelength of your favorite radio station if AM = X 103 and FM = X 106

  41. Doppler Effect • Changes the frequency and wavelength of light • Changing frequency of light changes color of light • Doppler Effect

  42. Red and Blue Shift • Red shift if object moves away • Violet or Blue shift if object moves toward

  43. Velocity of Doppler Effect • The speeds that you have to travel for the Doppler effect to be significant are very high but can be calculated using: vREL = ([fo – fS] / 2fS) X c

  44. Teacher Example • Determine how fast a car must be going for a red light to appear green.

  45. Teacher Example If police radar works at 24.15 GHz Determine how fast you are going if the return signal is 24.14 GHz

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