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Electromagnetic Waves

Chapter 24. Electromagnetic Waves. Background Notes: Light - what is it? Particles or Waves?. 200 year debate started in 1700’s. Supposedly settled in 1802. Finally resolved in early 1900’s. Before the 1700’s!. The Particle Theorists. Pierre Simon Laplace (1749-1827). Sir Isaac Newton

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Electromagnetic Waves

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  1. Chapter 24 Electromagnetic Waves

  2. Background Notes:Light - what is it? Particles or Waves? • 200 year debate started in 1700’s. • Supposedly settled in 1802. • Finally resolved in early 1900’s. Before the 1700’s!

  3. The Particle Theorists Pierre Simon Laplace (1749-1827) Sir Isaac Newton (1642-1727) Support later provided by: Jean-Baptiste Biot (1774-1862) Sir David Brewster (1781-1868)

  4. The Wave Theorists Rene DesCartesChristian Huygens Robert Hooke (1596-1650) (1629-1695) (1635-1703) Support later provided by: Thomas Young (1773-1829) Max Planck (1858-1947)

  5. Light. Waves or Particles? By 1727, Newton explained 3 particle properties. In 1802, Thomas Young demonstrated Diffraction and Interference, proving that Light was a wave.

  6. Thomas Young demonstrated diffraction and interference of light in 1802. Is this an example of chromatic aberration? LNK2LRN And remember, “Blue bends best!”

  7. Young was able to derive the equation for λ , the wavelength of light using similar triangles. x λ λ = x·d / L or λ = d·sinθn/ n LNK2LRN

  8. Today we know the wavelengths of the colors of visible light. ColorWavelength (nm) Violet410-440 Blue440-490 Green490-540 Yellow540-600 Orange600-630 Red630-770

  9. Today: Visible Light • Electromagnetic radiation that stimulates the retina of our eye. • We see objects due to reflected light. Two types of objects: • (1) Luminous - emit own light (ex. Sun) • (2) Illuminated - reflect light (ex. Moon)

  10. 4 Classic Attempts to Measure Speed of Light • Galileo (Italy) – Lanterns 1 mile apart, 1600 • Roemer (Denmark) – Moons of Jupiter, light crossing Earth’s orbit, 1676 • Fizeau (France)– Rotating Cogwheel chopping beam of light into pulses, 1849 • Michelson (USA) – Rotating Octagonal mirror reflecting light, 1878

  11. Galileo’s 1600 Lantern Experiment • Two people far apart, with covered lanterns. One uncovers his lantern, then the other immediately uncovers his on seeing the light from the first. • Result: Galileo said, “celeritas”, a Latin word, translated as “swiftness.” This is why we use c for speed of light. A Approx one mile B

  12. The Speed of Light by Roemer In 1676, Roemer found that the eclipses of Jupiter’s moons were off by 22 minutes when Jupiter was near Earth, compared to when Jupiter was farther from Earth. Diameter of Earth’s orbit = 3.0x1011 m Speed of light = 220,000 km/s. (Proven finite.)

  13. Fizeau’s 1849 Cogwheel Experiment Highlights of Fizeau’s experiment: • used a slit to produce a narrow beam of light • light travels through the spaces of a cogwheel • reflects off of a mirror • he adjusted the rotational speed of the cogwheel until the light passes through the next space on the wheel. Using this method, Fizeau determined that c =315,000 km/s.

  14. Michelson’s 1878 Rotating Mirror Experiment • German American physicist A.A. Michelson used a rotating octagonal mirror. • With this method, Michelson was able to calculate c = 299,792 km/s • Accepted as the most accurate measurement of c for the next 40 years. • Nobel Prize Picture credit

  15. POLARIZATION Canceling planes of vibration for light. Given by Malus’ Law Named after the French Physicist Etienne Louis Malus (1775-1812) HI! Polarizer analyzer So S S = Socos2θ

  16. Oscillating Electric and Magnetic fields Wave speed = c = 3.00 x 108 m/s in vacuum c=f  Wave Equation Electromagnetic Waves • Often just called ‘light’ waves, even though not all types are visible • They are transverse waves: • They can travel in a vacuum or through matter. • v=f 

  17. Francesco Grimaldi (1618-1663), professor of mathematics at the Jesuit college in Bologna, Italy noticed that edges of shadows were not sharp. (1st report of interference fringes and diffraction of light.) Later influenced Newton. LNK2LRN

  18. James Clerk Maxwell(1831-1879) First Cavendish Professor of Physics at Cambridge in 1871 LNK2LRN

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  23. QUICK QUIZ • Which one of the following scientists was not a wave theorist concerning light? (a) DesCartes (c) Hooke (e) Huygens (b) Newton (d) Young • Albert Michelson’s first attempt to measure the speed of light involved an 8-sided mirror apparatus rotating at 640 revolutions per second. One reflection was made to a parabolic/flat mirror 29.000 km away, which sent the pulse of light back again during the amount of time for only 1/8 of a revolution of the 8-sided mirror. Based on this data, what would result for the speed of light? (a) 3.0000 × 108 m/s (c) 2.9638 × 108 m/s (e) 2.9276 × 108 m/s (b) 3.0445 × 108 m/s (d) 2.9696 × 108 m/s

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  25. The Electromagnetic SpectrumA range of light waves extending in wavelength from radio waves to gamma rays. • Radio Waves - communication • Microwaves - used to cook • Infrared - heat • Visible Light - detected by your eyes • Ultraviolet - causes sunburn • X-rays - penetrates tissue • Gamma Rays – mutates cells LNK2LRN

  26. Electromagnetic Spectrum The End (or is it?)

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