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PHYS 140 Light & Color

PHYS 140 Light & Color. Course Outline Ch. 1: Physical Nature of Light Ch. 2: The Origin of Color Ch. 3: Colorimetry Ch. 4: Geometric & Vision Optics Ch. 5: Wave Optics Ch. 6: Light & Color in Nature Ch. 7: Color Vision. Review Scientific or “powers of ten” notation. Units.

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PHYS 140 Light & Color

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  1. PHYS 140 Light & Color

  2. Course Outline • Ch. 1: Physical Nature of Light • Ch. 2: The Origin of Color • Ch. 3: Colorimetry • Ch. 4: Geometric & Vision Optics • Ch. 5: Wave Optics • Ch. 6: Light & Color in Nature • Ch. 7: Color Vision

  3. Review • Scientific or “powers of ten” notation. • Units. • Algebra & geometry. http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/ http://physics.nist.gov/cuu/Units/units.html

  4. Problem Speed of light is 3 x 108 m/s. How long does light take to travel from the nearest galaxy, a distance of 2 x 1022 m? Express your answer in years. Answer: About 2 x 106 years… That means this galaxy is 2 x 106 light years away!

  5. The Scientific Notation (Continued) Sometimes, powers of 10 are replaced by prefixes. Examples: 1 1 unit 1,000 103 kilo (k) 1,000,000 106 mega (M) 1,000,000,000 109 giga (G) 0.001 10-3 milli (m) 0.000001 10-6 micro (m) 0.000000001 10-9 nano (n)

  6. On The Lighter Side… • 2000 pounds of Chinese soup: Won ton • 1 millionth of a mouthwash: 1 microscope • Time between slipping on a peel and smacking the pavement: 1bananosecond • Weight an evangelist carries with God: 1 billigram • Time it takes to sail 220 yards at 1 nautical mile per hour: Knot-furlong • 365.25 days of drinking low-calorie beer because it's less filling: 1 lite year • Half of a large intestine: 1 semicolon • 1000 aches: 1 kilohurtz • Basic unit of laryngitis: 1 hoarsepower • 1 million microphones: 1 megaphone • 1 million bicycles: 2 megacycles • 10 cards: 1 decacards • 1 millionth of a fish: 1 microfiche • 1 trillion pins: 1 terrapin • 2 monograms: 1 diagram • 8 nickels: 2 paradigms

  7. Chapter 1: The Physical Nature of Light What is sight?

  8. Early Theories • 550 – 400 BC (Pythagoras): • Traveling images • 200 BC – 40 AD (Euclid): • Visual Rays • 1000 AD (Alhazen): • Light = Physical thing • Eye = Detector of light

  9. Euclid’s Idea of Vision!

  10. Today Sight = Light + Eye/Brain • Direct (Source) • Transmitted or Reflected • Scattered Detector

  11. Some Properties of Light • Reflection (light can be reflected) • Refraction (light can travel in different transparent media) • Shadows (light travels in straight lines) • Dispersion (colors) • Speed…???

  12. Some Early Attempts to Measure the Speed of Light • Galileo (early 1600’s) • Flashing lanterns • Roemer (1670’s) • Indirect asrtonomical observations

  13. Galileo’s Attempt at Measuring the Speed of Light 15 km Note: Round trip transit time for the light signal would have been 0.0001 second!

  14. Diameter = 300,000,000 km Time delay between eclipses as observed from position 1 to position 2 was about 1000 seconds!

  15. Some Modern Methods for Measuring the Speed of Light

  16. Newton’s Particle Theory • Proposed in the mid 1660’s. • Prism experiments: Spectrum of colors White Light Prism Pinhole • White light contains all colors of the spectrum. • Colors can be mixed to get white light. • Objects appear colored because they reflect, scatter, • or transmit that color while absorbing others.

  17. Newton’s Particle Theory (Contd.) • Postulates: • Visible light consists of extremely small particles (corpuscles). • Light particles travel at enormous or infinite speed. • Colors correspond to different particle sizes. • Newton’s theory could explain all the known (at the time) properties of light. Dispersion White Light Spectrum Color Mixing

  18. Huygens’ Wave Theory • Proposed about the same time as Newton’s particle theory. • Postulates: • Visible light is a wave phenomenon. • Colors correspond to different wavelengths. • Drawback: Waves need a medium to travel. • How can light travel through vacuum? www.explorescience.com/activities/Activity_page.cfm?ActivityID=51

  19. Young’s Double Slit Experiment • Performed in the early 1800’s. • Demonstrated the “Interference Effect” with • visible light. Light must be a wave! • www.Colorado.EDU/physics/2000/schroedinger/two-slit2.html

  20. Waves • What is a “wave”? • Any periodic disturbance that repeats itself in time and travels through space with definite speed. • Examples: Water waves, waves on a string, sound, visible light, x rays, microwaves…etc. • Note: Most of the information comes to us in the form of waves! • Waves transmit energy! http://surendranath.tripod.com/Applets.html

  21. Wave Characteristics • Waves consist of periodic motion in time. This repetition rate is measured in terms of time period or frequency. • Waves transport energy. “Strength” or “loudness” of the wave is measured by its amplitude. • Waves travel through space. The distance traveled during 1 time period is called the wavelength.

  22. Wave Speed • Wavelength is the distance traveled during one time period. By definition, or Note: Above relations are true for any wave. For a wave with constant speed, wavelength is inversely proportional to its frequency.

  23. Review Problems • If the frequency of a wave is 10 Hz, what is its time period? • A given wave has a wavelength of 10 cm and a frequency of 200 Hz. What is the speed of the wave? • The speed of sound on planet Mongo is 800 m/s. A normal person can hear sound in the frequency range of 20 Hz to 20 kHz. What wavelength range is this? • A typical AM radio frequency is 1000 kHz. What wavelength does this correspond to? 0.1 s 20 m/s 40 m – 0.04 m 300 m

  24. Properties of Waves • Superposition Principle: • Two or more waves can overlap in space or time and produce a single net disturbance. • This principle gives rise to many interesting effects! http://surendranath.tripod.com/Applets.html

  25. Interference Effect • Constructive Interference • Amplitudes are added. • Result: Single wave with large amplitude. = Overlapping waves are “in phase”

  26. Interference Effect (Contd.) • Destructive Interference • Amplitudes are subtracted • Result: Single wave with small or zero amplitude. Overlapping waves are “out of phase” www.colorado.edu/physics/2000/index.pl www.physics.nwu.edu/ugrad/vpl/waves/superposition2.html

  27. Particle-Wave Controversy (1800’s) Light • Waves? • Could explain: • Reflection • Refraction • Interference • Could not explain: • Polarization • Travel without medium • Particles? • Could explain: • Reflection • Refraction • Polarization • Could not explain: • Interference

  28. The Electromagnetic Wave Model • Proposed by Maxwell in early 1870’s. • An electromagnetic field exists around a • moving charge. (Field can exist in a vacuum!) Shake me! + http://webphysics.davidson.edu/Applets/Retard/Retard.html • Maxwell’s calculations showed that if this charge is oscillated, the disturbance in the field travels at the speed of light! Is light an electromagnetic wave?

  29. Electromagnetic Wave Theory • Postulates: • Light is an EM wave produced by accelerated charges within atoms. • It consists of oscillating electric and magnetic fields that are mutually perpendicular. • All EM waves travel at the speed of light, but they have different wavelengths (colors) and frequencies.

  30. The Electromagnetic Wave http://employees.oneonta.edu/labroos/animations/phe/emwave.htm

  31. The Electromagnetic Spectrum 3 x 108 m/s = Wavelength x Frequency www.Colorado.EDU/physics/2000/waves_particles/index.html

  32. Electromagnetic Wave Model (Contd.) • This model is adequate in explaining the • “wavelike” characteristics of light. • Drawbacks of the EM theory: • Blackbody radiation. • Photoelectric effect. • How does an EM wave carry energy?

  33. Blackbody Radiation • Shape of the blackbody radiation curve could not be explained by the classical theory. • ww2.unime.it/dipart/i_fismed/wbt/ita/physlet/blackbody/corponero.htm

  34. Photoelectric Effect • Presence of a cut-off frequency could not be explained by the classical theory. • http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm Electrons being ejected from a metal surface by absorption of light

  35. The Quantum Idea • Initially proposed by Planck in 1890’s to explain “blackbody radiation”, and later adopted by Einstein in 1905 to explain the “photoelectric effect”. • Mechanical wave: Energy is related to its amplitude, so it can carry any continuous amount! • EM wave: Planck proposed that energy is related to its frequency, so it can only carry discreet amounts.

  36. Gravitational Analogy of the Quantum Idea Gravitational potential energy is proportional to the height! Ramp: Energy = Any amount Steps: Energy = 1, 2, ..Quanta

  37. The Quantum Model • Postulates: • EM waves carry energy in “bundles” or “packages” called PHOTONS. • Photon energy is given by, • Note: • Planck’s constant, h = 4.1 x 10-15 eV-sec. • Where 1 eV = 1.6 x 10-19 Joules of energy.

  38. Review • Calculate the photon energy in eV of the following waves: • 100 MHz radio wave • 500 nm visible light • 0.1 nm x-ray 4.1 x 10-7 eV 2.46 eV 12,300 eV

  39. Wave-Particle Duality Light Waves? Particles (Photons)? • Wave-like characteristics: • Shows interference. • Particle-like characteristics: • Has momentum. • Exerts pressure (comet’s tail always points • away from the sun!). • Note: Particles like electrons and neutrons show wave-like characteristics!

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