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Light: An Introduction

Light: An Introduction. Or You are the light of the world. Light in general. Electromagnetic wave Transverse wave Caused by accelerating or vibrating electric charge. Brightness follows inverse square law Intensity ~ 1/d 2 Doubling distance means 1/4 th the brightness

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Light: An Introduction

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  1. Light: An Introduction Or You are the light of the world

  2. Light in general • Electromagnetic wave • Transverse wave • Caused by accelerating or vibrating electric charge. • Brightness follows inverse square law • Intensity ~ 1/d2 • Doubling distance means 1/4th the brightness • Dual nature of light

  3. Types of Light: The EM Spectrum • All EM waves alike • Produced by accelerating electric charge • Travel at 300 000 km/s in a vacuum • Waves differ in • Frequency • Wavelength • Energy

  4. Transparent vs. Opaque • Transparent • Incident light is not at a natural frequency of the substance. • Small forced vibrations pass the energy from molecule to molecule • Energy is transmitted through the material • Opaque • Incident light is at the natural frequency of the substance • Resonance causes large vibrations which crash one molecule against another creating heat • Energy is not transmitted • No sun tan in green house example or how to stay married • Glass opaque to UV light • Glass transparent to visible light

  5. Speed of Light • 300, 000 km/s =300 000 000 m/s =3 x 108 m/s = 186,000 mps • Roemer calculation • Michaelson calculation • Cosmic speed limit • In media • Speed less than that in vacuum • speed~ 1/density • Speed~ temperature 1900-1930 1647

  6. Cosmic Distances • Light year is distance light travels in one year • 6 trillion miles • Sun nine light minutes away • Pluto 14 light hours away • Nearest star 4.3 LY away • Center of galaxy 30 000 LY away • Next galaxy 2.52 million light years away • Looking into space is looking back into time • Lots of empty space out there

  7. Realms of the Universe Image courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002

  8. Earth • Planet where we all live • Comprised primarily of rock • Spherical in shape • 12,700 km in diameter • It would take 17 days to circumnavigate the globe driving a car at 100 km/hr • At the speed of light, it would take 0.13 seconds to go all the way around Earth.

  9. Sun • Star that Earth orbits • Composed primarily of hydrogen and helium gas • Uses nuclear fusion in its core to generate heat and light to allow itself to resist the crushing weight of its own mass • Spherical in shape • 1.39 Million km in diameter

  10. Earth & Sun • The Sun’s diameter is 109 times greater than that of Earth • Over 1 million Earths would fit inside the Sun’s volume • Earth orbits the Sun at an average distance of 150 million kilometers. This distance is called an Astronomical Unit (AU) • It would take 11,780 Earths lined up side to side to bridge the 1 AU between Earth and Sun.

  11. The Solar System • 8 planets, thousands and thousands of dwarf planets and asteroids, trillions of comets and meteoroids • Mostly distributed in a disk about the Sun • Sun blows a constant wind of charged gas into interplanetary space, called the Solar Wind Boundary between Solar Wind and interstellar space at 100 AU from the Sun (200 AU diameter)

  12. The Solar Neighborhood • The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood. • Here stars move generally with the Sun in its orbit around the center of the Galaxy • This region is inside a large bubble of hot interstellar gas called the Local Bubble. Here the gas temperature is about 1 million degrees Kelvin and the density is 1000 times less than average interstellar space. To Center of Galaxy The image is 390 light-years across. Direction of Galactic Rotation

  13. The Milky Way Galaxy The Milky Way Galaxy is a giant disk of stars 160,000 light-years across and 1,000 light-years thick. The Sun is located at the edge of a spiral arm, 30,000 light-years from the center It takes 250 Million years for the Sun to complete one orbit There are over 100 Billion stars in the Milky Way The Spiral arms are only 5% more dense than average, and are the locations of new star formation

  14. The Local Group • Contains 3 large spiral galaxies--Milky Way, Andromeda (M31), and Triangulum (M33)—plus a few dozen dwarf galaxies with elliptical or irregular shapes. • Gravitationally bound together—orbiting about a common center of mass • Ellipsoidal in shape • About 6.5 million light-years in diameter

  15. A cluster of many groups and clusters of galaxies • Largest cluster is the Virgo cluster containing over a thousand galaxies. • Clusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expands. • The Local Supercluster gets bigger with time • It has a flattened shape • The Local Group is on the edge of the majority of galaxies • The Local Supercluster is about 130 Million light-years across The Local Supercluster

  16. The Universe • Surveys of galaxies reveal a web-like or honeycomb structure to the Universe • Great walls and filaments of matter surrounding voids containing no galaxies • Probably 100 Billion galaxies in the Universe The plane of the Milky Way Galaxy obscures our view of what lies beyond. This creates the wedge-shaped gaps in all-sky galaxy surveys such as those shown here.

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