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Week 5 Astronomy Notes

by: Raleigh Zook

Week 5 Astronomy Notes ASTR 1210

Marketplace > University of Virginia > Astronomy > ASTR 1210 > Week 5 Astronomy Notes
Raleigh Zook
GPA 3.55

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About this Document

These notes go over light and telescopes and their properties.
Introduction to the Sky and Solar System
Remy Indebebetouw
Class Notes
astronomy, Science, Telescope, space, light
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This 6 page Class Notes was uploaded by Raleigh Zook on Thursday February 25, 2016. The Class Notes belongs to ASTR 1210 at University of Virginia taught by Remy Indebebetouw in Spring 2016. Since its upload, it has received 21 views. For similar materials see Introduction to the Sky and Solar System in Astronomy at University of Virginia.


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Date Created: 02/25/16
Telescopes       • Simplest  possible  energy-­‐forming  device:  Eye   o Pupil,  lens,  retina,  optic  nerve  à  Brain   o Muscles  on  lens  of  the  eye  which  pull  to  adjust  the  focus/lens   Refraction   • When  light  goes  from  one  medium  to  another  substance,  changes  speed,   which  therefore  can  change  direction,  as  well   o Bends  light   § E.g.  when  going  into  window  (light  slows  down)   o Light  approaches  at  an  angle   o E.g.  refraction  at  sunset   • Can  cause  light  to  converge  in  order  to  focus   o E.g.  eye  using  this  in  order  to  focus  light   • Wavelength  is  smaller  –  Object  that  the  light  interacts  with  can  make  light  a   “ray”   o Can  cause  parallel  light  rays  (To  converge  to  a  focus  point)   Convex  Lens   • Convex  is  caved  out,  while  concave  is  caved  in   • When  a  ray  is  perpendicular  to  the  lens,  then  it  bends  to  a  focal  point:   o Focal  length  is  twice  the  radius  of  a  curvature     o Thicker  lens  is  a  stronger  lens—shorter  focal  length  though     § The  same  object  will  end  up  closer,  so  you  will  focus  better  on   more  distant  objects   • When  its  source  is  infinity,  the  image  is  at  a  focal  length   • Central  ray—It  is  straight;  A  parallel  ray  goes  through  a  focus  (Focal  point  is   where  parallel  rays  intersect);  Going  through  the  focus,  it  is  parallel   • Image  is  upside  down   Image  Formation   • Real  objects,  however,  are  not  at  an  infinity  (except  for  stars)   • Focal  plane:  Light  comes  into  focus  from  different  directions   • Change  properties  of  lens  to  see  things  further  away   • Use  multiple  lenses  together  (Instead  of  varying  properties  of  one  lens)   o Successive  variance—sequential     Mirrors   • Similar  to  a  convex  lens,  a  concave  mirror  (convex  mirror  and  concave  lens)   focuses  light  and  bounces  it  back   • Focus=R/2  (Instead  of  2R)   • The  angle  of  incidence  is  equal  to  the  angle  of  reflection     • Real  image  (where  the  light  is  physically  being  brought  to  focus)  are  upside   down   o Light  is  converged   o If  diverged  (light  in  not  real  focus—all  directions),  but  brain     • The  eye  forms  an  image  (using  a  convex  lens)   • The  focal  length  is  the  location  of  the  image  for  the  source  at  infinity   • Convex  lenses  and  concave  mirrors:  Form  real  images—Opposed  t  how   concave  lenses  and  convex  mirrors  form  virtual  images   Refracting  Telescopes   • Are  required  to  be  long  with  a  very  heavy  lens     • Uses  lenses  to  focus  light   Reflecting  Telescopes   • Uses  mirrors  to  focus  light   • Most  telescopes  nowadays  are  reflecting  telescopes   • Greater  diameters   • E.g.  Keck  telescope   Light Week 5 ASTR 1210 Defining Light • Light is electromagnetic energy—There’s an electric field o Changes to create a magnetic field and then changes to become electric wave—back and forth (Passing energy back and forth) • Light can push; light can have pressure • Wavelengths or frequencies are used to characterize light o Longer wavelengths = Red o Shorter wavelengths = Purple • Wavelength is the distance between two adjacent crests (or troughs) • Its amplitude is half the difference between height in a crest and trough • Frequency is the amount of crests passing through a point each second—Measured in hertz (Cycles per second) o Blink rate is frequency • Speed: How fast the crest and trough pattern moves • Wavelength x Frequency = Speed of Wave • Theories: o Ptolemy and Euclid state it is a ray traveling from one’s eye to an object o Aristotle: (The reverse) Object to the eye o Descartes: Waves traveling in plenum o Newton: Particles o A wave o Einstein: Photoelectric effect—a particle • Constructive versus destructive interference o Wave strengths adding together or cancelling each other out • Michelson-Morley experiment • Light is waving in/with itself Light Speed • Galileo tried to measure speed of light with a friend on a hill • Timing Jupiter’s moons’ eclipses • NOW: Interferometry is used Photoelectric effect • Red light on block of metal—nothing happens o If you make more energy in a wave (make it bigger), still nothing happens • Blue light on block of metal—then electrons come out of the metal moving fast • Green light: Some electrons • Light comes in discrete packets—Energy increases with frequency from red to blue • Wave/Particle Duality Light Interactions • (Specular) Reflection o Incoming light one direction, outgoing light one direction o Angle at which is comes in, is the same angle it leaves o Setting up two mirrors at 90 degrees, it will send light/signal right back where is came (Corner/Retro Reflector) § E.g. buoys § E.g. On moon—set of a few dozen corner reflections; lasers to measure how far away it is (3.8 cm away from Earth each year) o Possible to change angle of polarization through reflection (Only get one, other gets absorbed) § E.g. Polarized sunglasses—Cuts the glare because the light coming off the reflective light is more polarized than light in generalà Partially dim preferentially • Transmission o Goes through o Atmospheric Transmission § Some wavelengths/colors that the absorption or scattering is so bad that it doesn’t get to us (gamma rays, X rays, ultraviolet light = 100% atmospheric opacity) • Blocked by upper atmosphere (observed from space) § Visible light is observed from Earth with only some atmospheric distortion § Infrared spectrum (observed from space) § Radio waves (observed from Earth) § Radio waves that are the longest wavelenths are blocked o Angle Dependent Transmission § E.g. Prisms; Rainbows • Best time to see rainbow is sun lower in the sky behind you, because the sunlight goes over head, hits raindrops and in the water the light bends and bounces off the back of the raindrop (Red and blue light coming out at different directions) § Different wavelengths • Scattering o Bounces off in all directions—Diffuses § E.g. Cinema § E.g. Cosmic microwave background • Similar to light coming through clouds on a cloudy day (how light is still visible) o Polarization § Waving in a perpendicular direction § Light is 3D and there are two ways to travel perpendicular (wiggle) to the direction of destination • Magnetic energy and electric energy • Can go through or not § Star emitting light and the light bounces off dust and comes to us o Selective Scattering § E.g. Red dye on a chair (absorbed all the blue light and scattered off red light); Leaves color changing—Absorbing all red light and blue (makes it green in summer), Absorbing green during fall (makes multitude of colors) § Why is the sky blue: Scattered better towards an individual (Same molecules as sunset, however deals with geometry) • White light comes directly toward us from the sun § Why is the sunset red: Blue light scattered better in a different direction • Appears red due to light coming directly from the sun • Absorption o Does not go through completely—becomes absorbed o Selective absorption: Only some wavelengths go out § E.g. Stained glass windows o Color = Wavelengths Emission 1. “Black Body” • Emission of a heated object is independent of the object’s properties o Warm it up, emits more light • Radiation intensity and wavelengths • Warmer light emits more blue light relative to the amount of red light • Cooler light emits more infrared light and omitting red—“Red hot” • White is hottest, orange is hotter, red is least hottest (Color of flames) • Hotter = bluer according to Wein’s law Peak wavelength = Constant/ Temperature Constant= 0.29cm K/Kelvin Use this to determine temperature of stars • Blackbody Radiation o Hotter: more energy emitted and peak moves bluer (shorter wavelength) o Brighter and bluer • Stefan Boltzmann Law: ▯ Energy per area = Constant x ???????????????????????????????????????????? Reason why sunspots are visible—temperature changes energy levels 2. Lines • Selective absorption (e.g. chlorophyll and different colors of leaves) o Inverse • Gas emitting at specific wavelengths (Instead of a continuous distribution), which provides a spectral ‘fingerprint’ (Identifies what gases are present, even in outer space) • Atoms both emit and absorb light (at specific wavelengths). These specific wavelengths are unique for each element. Atom emits light only at one wavelength because of quantity— quantum mechanics o Electrons are not allowed to orbit at any rate they want—limited to rate of energy. If it goes from higher energy to lower energy, it produces a precise wavelength. Emission line is produced due to this • Use specific energies that molecules are sensitive to in order to create a continuous spectrum, emission spectrum, and absorption spectrum. Composition from lines, temperature from broad color. • “Redshifted” o There is a lower frequency and lower energy, which causes it to move away; Shifted to the right of the standard • “Blueshifted” o There is higher frequency and higher energy, which causes it to move toward you; Shifted to the left of the standard Side Notes: • Our eyes are limited to what colors are seen. There is light with longer and shorter wavelengths that are not visible to the human eye. • Red radiation is a good detector of heat (infrared thermal properties); Cold-blooded doesn’t emit this (blue) • Infrared Universe • Radio Universe (e.g. Jupiter) o Innermost moons lose material and get caught in magnetic field § Interaction of material from innermost satellite with this field • Ultraviolet wavelength is shorter than infrared • We know/can find out: o Brightness and Luminosity o Temperature through Blackbody o Composition through spectral lines o Velocity through the Doppler shift Quantum Mechanics Wave/Particle Duality • Light particles are “normal” • Neon atoms one at a time—Interference pattern, two slits o If you send one through at a time, it goes through both slits (electron is everywhere), which interferes with itself when not observed, but when observed it only goes through one slit and doesn’t interfere anymore (forcing it to be somewhere) • All objects have a possibility of distribution, a probability of weight function • Schrodinger’s cat o Cat in a box, putting poison in box with a random event inducing the poison § If the material does not decay; the cat lives § If the material does decay, then it has been killed o Until you look, the cat is both dead and alive o Once observation is made, then force the universe to be in a certain state • Wave functions (distribution) collapse due to observations o Once you look, there is beta detection o Objects technically do not exist until observed • Quantum entanglement o If two particles (who both have wave properties) happen to interact, their wave functions get synchronized, and stay like that even when measured separately § MRI, Quantum detectors • Quantum: cryptography (cannot copy it it without disturbing it), computing, teleportation (entanglement)


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