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AS101 Final Exam Review Notes

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Patrick Mc Graw

Final: Review Notes Format - 90 minutes, 65 MC + 2 Bonus - Cumulative, more emphasis on material covered post-midterm (slightly) - Coverage: Chapters 1-4, 5.2, 5.4, 12-14 (Chapter 15: bonus questions) - No calculations other than recognizing a number in scientific notation Main + Secondary Ideas *Secondary Ideas italicized Chapter 4 - Telescopes in general - Electromagnetic spectrum - Challenges to observation - Main functions of a telescope - Different types of light and how they are used in astronomy (radio, infrared, x-ray, etc.) - Techniques (interferometry, adaptive optics, etc.) Chapter 5 - Spectra as our main source of information - Continuous vs. line spectra & what they tell us - What the Doppler effect tells us - Difference between 2 types of line spectra (emission and absorption) o How they are formed and how they are related to physics of atoms and light Chapters 12-14 - Overall structure of the solar system - How does what we know about it fit into our understanding of how it formed? - How do we learn about planets around other stars? - What do we know about individual planets, moons etc. o How have we learned it? - How do we understand some of the reasons planets and moons ended up as they are now? Chapter 4: Astronomical Telescopes and Instruments: Extending Humanity’s Vision Telescopes - Built optical telescopes to gather light and focus it into sharp images - 2 types of telescopes: o Refracting telescope  Lens bends (refracts) light as it passes through the glass, brings it to a focus to form an image  Main lens: primary lens  has serious disadvantages:  suffer from optical distortion that limits usefulness o when light refracted, shorter wavelengths bend more than longer wavelengths (result: colour blur around images, called chromatic aberration)  glass in primary lenses must be pure and flawless because light passes all the way through it o weight of lens can only be supported around outer edge o Reflecting telescope  A mirror forms an image by bouncing light  Main mirror: primary mirror  Less expensive to create mirror, no chromatic aberration, so these tend to be built more  Advantages:  No chromatic aberration  Mirrors can be supported from behind, not just at the edges (less sagging)  Easier to polish single surface than to make a lens free of defects all the way through - Both telescopes form a small, inverted image, so a lens called the eyepiece used to magnify image and make it convenient to view - Focal length = distance from lens/mirror to image it forms of a distant light source (e.g. star) - Surfaces of lenses/mirrors must be shaped and polished to have no irregularities larger than wavelength of light - LARGER TELESCOPES: greater light-gathering power, better angular resolution - ADVANTAGE OF PLACING TELESCOPES IN SPACE o Eliminates atmospheric distortion o Allows you to see wavelengths that get absorbed by atmosphere (gamma rays, far IR, etc.) - Telescopes located on Earth must move continuously to stay pointedat a celestial object (Earth turns on its axis) – sidereal tracking (sidereal  stars) Powers of a Telescope 3 powers to help human eyes, most important 2 depend on diameter of telescope 1. Light-gathering power - Ability of telescope to collect light & concentrate it to make faint objects visible - The wider, the more light it catches - Proportional to the square of primary’s diameter o 2m diameter has 4x the light-gathering power of a 1m telescope 2. Resolving power - Ability of telescope to reveal fine detail - Limited by diffraction fringe (rings of blurring around points of light) o 2m telescope has diffraction fringes ½ as large, so 2x better resolving power than 1m telescope o Can be improved using interferometry, connecting 2+ telescopes. Has a resolving power equal to a telescope as large as maximum separation between the individual telescopes - Limited by chromatic aberration (problem with lenses, not mirrors, where different colours of light focus at different points) o Not a problem with reflecting telescopes - Atmospheric distortion, aka seeing: irregular bending of light as it passes through layers of the atmosphere. Causes stars to twinkle and causes blurred images o Improve by observing from mountains and dry location, or from space o Or adaptive optics: rapid computer calculations adjust the telescope optics and partly compensate for the distortions 3. Magnifying power - Ability to make an image larger - - A telescope with primary mirror with focal length of 700mm and eyepiece focal length of 14mm has magnifying power of 50. (Textbook says 503…) - Magnifying image doesn’t make it sharper necessarily, different from resolving power - Can be fixed by changing eye piece Special Techniques Interferometry - Linking multiple telescopes together to reduce diffraction effects & improve angular resolution - Having a telescope as wide as the distance between the individual telescopes - Mostly used for radio telescopes, but some visible light telescopes as well Adaptive Optics - Using a computer-controlled deformable mirror to partly compensate for atmospheric distortion - Improves seeing for ground-based telescopes Electromagnetic Spectrum - The types of electromagnetic radiation arranged in order of increasing wavelength - Visible light is only small part of the spectrum -9 -10 - Nanometre (nm) is one billionth of a metre (10 m), Angstrom (A) is 0.1nm or 10 m *The following are in order from shortest wavelengths to longest wavelengths Gamma Rays X-Rays Ultraviolet (UV) - Shorter than violet rays Visible - Rainbows are spectra of visible light o Colours of visible light have different wavelengths: -7 -7 o Red = longest (700nm/ 7x10 m), Violet = shortest (400nm/ 4x10 m) - Average wavelength of visible light: 0.0005mm - 50 light waves would fit end to end across thickness of a sheet of paper Infrared (IR) - Beyond the Red end of visible range - Wavelengths range from 700nm-1mm (eyes are not sensitive to this, but skin can sense some heat) - “invisible light” Microwave - Used for radar/long distance telephone communications - 1mm to a few cm Radio - Longer waves than IR radiation - Radio radiation used for AM has wavelengths of a few hundred metres - FM, television, military/governmental/amateur radio transmission wavelengths range from few 10’s of cm to few 10’s of m HOW THEY ARE USED IN ASTRONOMY - Light behaves as a wave but also behaves like a particle of light under certain conditions (photon) o Bundle of electromagnetic waves - Shorter wavelength photons carry more energy, longer wavelength photons less o X-ray photon carries more energy, visible light less - Astronomers are interested because it carries almost al available clues to the nature of planets, stars and other celestial objects - Earth’s atmosphere is opaque to most electromagnetic radiation - To study the sky, must look through an atmospheric window of the spectrum (visible light, short-wavelength IR and some radio waves) Chapter 5: Sun Light and Sun Atoms Kirchhoff’s Laws (3 basic types of spectra) 1. The Continuous Spectrum (thermal) - (or a thermal, or a blackbody radiation) - A solid/liquid/dense gas excited to emit light will radiate at all wavelengths - Produced by hot, dense object - Relative amount of each wavelength depends on temp. (Hotter  more short wavelengths) - Examples: a standard incandescent light bulb, the sun, a glowing heating coil on the stove, a human body - STEFAN-BOLTZMAN LAW: o Hotter objects emit more radiation than cooler ones - WIEN’S LAW: o Hotter objects emit photons with higher than average energy o Wavelength of peak intensity shifts toward lower wavelengths as temp. increases o This means we can figure out temperature of star by looking at thermal spectrum 2. The Emission Spectrum (line) - Low-density gas excited to emit light will do so at specific wavelengths - Depends on type of atom or molecule 3. The Absorption Spectrum (line) - Low-density gas lit from behind absorbs particular wavelengths - Depends on type of atom Line Spectra - Electrons in an atom can only orbit in specific orbits with particular energies - Can only absorb or emit energy by moving from one level to another - Under normal conditions, atom spends time in ground state (lowest energy state) o If photon with right energy comes along, electron can absorb energy and bounce up to an excited state o Energy is emitted again as atom falls back to ground state What the Spectra tells us Continuous - Tells us about object’s temperature - Hotter objects emit more light overall - More of that light is shorter wavelengths/higher frequencies Line - Tell us about chemical composition - Provide reference point for Doppler Effect The Doppler Effect - An observed change in the wavelength of radiation caused by relative motion of a source and observer - Astronomers use it to measure the speed of blobs of gas in the Sun’s atmosphere toward/away from Earth, as well as speeds of entire stars and galaxies - Small shifts in the wavelength of electromagnetic radiation - Can measure speed of star moving toward/away from you by measuring size of Doppler shift of its spectral lines - Light from stars in distant galaxies show a REDSHIFT, indicating that galaxies are moving away. Universe is expanding! Doppler Shift of Sound Waves - Pitch of sound determined by wavelength - Long wavelengths = low pitches, short wavelength = high pitch Doppler Shift of Light Waves - Light source approaching you = shorter wavelength, blueshift - Light source moving away from you has longer wavelength, redshift - Red and blue refer to relative direction of the shift, not actual colour Chapter 12: The Origin of the Solar System Parts of the Solar System Main Objects - Sun - Planets - Meteoroids, Asteroids, Comets and Dwarf Planets The Sun - Includes most of the matter in the solar system - Mostly hydrogen & helium The Planets - Terrestrial planets are small, rocky and closer to the sun o Mercury, Venus, Earth, Mars - Jovian planets are large, gassy and farther from the sun (contain more hydrogen, helium, etc.) o Jupiter, Saturn, Uranus, Neptune o Have more moons than terrestrial o All have rings, Saturn’s are most noticeable - My Very Excellent Mother Just Served Us Noodles o Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune - PLANETS are large enough (and therefore gravity stro
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