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White light is composed of all the colors of the rainbow

White light is composed of all the colors of the rainbow. Rainbow of colors of the visible spectrum can be split by a prism. The visible spectrum. Red Orange Yellow Green Blue Indigo Violet. A single color can’t be split again. Spectrum can be recombined into white light.

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White light is composed of all the colors of the rainbow

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  1. White light is composed of all the colors of the rainbow • Rainbow of colors of the visible spectrum can be split by a prism

  2. The visible spectrum RedOrangeYellowGreenBlueIndigoViolet

  3. A single color can’t be split again

  4. Spectrum can be recombined into white light

  5. Light is electromagnetic radiation

  6. Higher frequency

  7. What is the speed of light? How do we know?

  8. Measuring the speed of light Light takes over 16 minutes to travel the diameter of earth’s orbit (2 AU)

  9. Speed of light is the universal speed limit • No physical object can travel at the speed of light • No information can travel faster than light • We can’t find out about the universe faster than c • Distance is a time machine!

  10. Light behaves as particles and as waves • Particles of light are called photons • Energy of a photon increases with shorter wavelengths E = h c / λ

  11. Most electromagnetic radiation is blocked by atmosphere • Good for life on earth, but bad for observing the universe • Much of the observation of the universe must take place from space • Visible light and radio waves are biggest “windows” in atmosphere

  12. Types of telescopes • Refracting telescopes focus light with an objective lens • Reflecting telescopes focus light with an objective mirror

  13. Refracting telescopes • Bend light to collect & magnify

  14. Refracting telescopes are made with two lenses Image is inverted in eyepiece!

  15. Light gathering power increases with aperture

  16. Aperture also increases resolution Magnification does no good if resolution is poor!

  17. Limitations of refracting telescopes • Spherical aberration • Chromatic aberration • Distortion of lens by gravity • Unwanted refractions (imperfections in glass) • Glass blocks some light (especially outside visible, e.g. UV)

  18. Spherical aberration

  19. Chromatic aberration (same effect that makes prism separate colors)

  20. 36” telescope is 2nd largest refractor in the world Lick Observatory,Mt. Hamilton • Severely impacted by light pollution from Silicon Valley

  21. Reflecting telescopes • Avoid many of the problems of refracting telescopes • Can be made much larger • up to 200” single mirror • much larger with new technology • up to 10 meter Keck telescopes

  22. No chromatic aberration Easier to make large mirrors that don’t distort Still have to correct for spherical aberration (or use parabolic mirror) Reflecting telescopes use a mirror to focus light

  23. Type of refactors a) Newtonian c) Cassegrain b) prime focus d) coudé

  24. Astronomical observations from surface of the Earth are limited • Turbulent air causes twinkling and poor “seeing” (lower resolution) • Most non-visible wavelengths are blocked by atmosphere • Light pollution is increasing problem • Space-based observatories don’t have these limitations (but cost $$$)

  25. Light pollution

  26. Space-based observatories • Hubble Space Telescope • 2.4 meter orbiting telescope • Observes in visible, near-IR, UV • Many others for IR, UV, X-ray, gamma ray

  27. New technology is revolutionizing ground-based astronomy • Active optics • Adaptive optics • Segmented mirrors • Optical interferometry

  28. Observing and recording the sky • First observations were made directly through telescopes, recorded by hand sketches • Photographic plates & film allowed long time exposures to capture greater amounts of light • CCDs now record images digitally

  29. M27 (Dumbell Nebula) seen through an amateur telescope

  30. Amateur long exposure photograph

  31. Radio Astronomy • First radio signals from space detected in 1932 • Can be done from the ground • Collecting dishes need to be large for long-wavelength radio waves

  32. Infrared (IR) telescopes • Infrared Telescope Facility (IRTF), Mauna Kea (NASA) • Infrared Astronomical Satellite (IRAS) • Space Infrared Telescope Facility (SIRTF) to be launched this spring

  33. Ultraviolet (UV) astronomy • International Ultraviolet Explorer (IUE) 45 cm • Far Ultraviolet Spectroscopic Explorer (FUSE)

  34. X-rays and gamma rays • High-energy photons cannot be focused like light • Very specialized orbiting instruments • Chandra X-ray observatory • Compton gamma ray observatory

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