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Discover the Universe Week 4

by: Jocelyn

Discover the Universe Week 4 AST 1002

Marketplace > University of Florida > Science > AST 1002 > Discover the Universe Week 4
Discover the Universe
Reyes, Francisco J

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About this Document

This week continued Chapter 2: Light & Matter briefly and delved into Chapter 3: Telescopes, The Tools of Astronomy. We learned about the different types of telescopes, how our eyes form an image, ...
Discover the Universe
Reyes, Francisco J
Class Notes
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This 10 page Class Notes was uploaded by Jocelyn on Saturday September 19, 2015. The Class Notes belongs to AST 1002 at University of Florida taught by Reyes, Francisco J in Summer 2015. Since its upload, it has received 31 views. For similar materials see Discover the Universe in Science at University of Florida.


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Date Created: 09/19/15
Tuesday September 15 2015 Discover the Universe Week 4 Chapter 2 Light amp Matter Cont d How does the electron get the energy it needs to become excited Collisions between atoms can excite electrons to higher energy levels Passing an electron current applying high voltage to a low density gas will make atoms collide Light can behave as a particle Light energy must be carried in packets called photons Einstein was awarded the Nobel Prize in 1921 for his theory of photoelectric effect The effect can be explained if lights considered as a particle photons Photon energy 1wavelength Photon energy frequency Light Intensity of photons arrivingsecond the intensity will increase with more photons per unit of time Low energy photons cannot cause electron ejections High energy photons cause ejection of elections can ionize an element Quantum Mechanics Atoms can only absorb or emit photons with energies exactly equal to the energy difference between electron orbits delta E The energy of a photon is related to the wavelength Energy of a photon E h f h c lambda f c ambda h is the Planck s constant The Hydrogen Atom Atoms of different elements have unique energy level structures Every electron transition corresponds to a unique wavelength Tuesday September 15 2015 Ionization ejection of electrons Part of the lines is the Balmer series of Hydrogen are in the visible part of the spectrum Every element atom emits or absorbs a particular set of lines Bohr s Hydrogen Atom ectron l O l l proton quotl 39 In Modern Quantum Mechanics Electrons are not just particles but also waves without exact locations unlike in Bohr s model Doppler Effect Motion along the line of sight radial motion produces a Doppler effect Light emitted by a moving object also presents a Doppler effect vc change in wavelength wavelength change wavelength shift wavelength at restwavelength at rest v is the velocity c is the speed of light change in lambda wavelength shift wavelength rest Tuesday September 15 2015 If it s close to you blue part of the spectrum if it s moving away from you red part of the spectrum BLLIESHIFT Important If the body emitting the Balmer series is receding moving away from the observer the lines are shifted to the red part of the spectrum The spectrum is said to be red shifted and vice versa for blue Obtaining the rotation of an object from the width of the Doppler Lines We will assume that the object isn t approaching or receding from the observer it is only rotating If the object is rotating the side approaching the observer will be blue shifted The side moving away from the observer will be red shifted The line emitted from the center will have no shift as a consequence the line will be wider than it would if the object had no rotation The rotation rate of the object can be determined by measuring the width of the spectral lines Thursday September 17 2015 The Zeeman Effect A single emission line can split into two or more under the presence of a magnetic field The presence of magnetic field splits the energy levels of at atom What can we learn from spectroscopy The chemical composition by matching the spectral lines with laboratory spectra of atoms The temperature by matching overall spectral shape with blackbody curve Thursday September 17 2015 The lineofsight velocity by determine the Doppler shift The rotation rate by measuring broadening of spectral lines due to Doppler shift The pressure of the gas in the emitting region due to broadening of spectral lines The greater the pressure the broader the line The magnetic field Chapter 3 Telescopes The Tools of Astronomy How does your eye form an image Light is refracted bent by the lens converging lens and forms an image in the retina Cross section of Human Eye Lens Ins quot39 gt v v V 14 39l l x 39 39I 7 l39i l l Iquot Cornea Retina Inverted image of object I s quot Optical nerve Pupil Liqliid Object Ligaments Jelly Cullimy f Muscle What is refraction Refraction is the bending of light when it passes from one substance into another a substance with a different refraction index Example from air to glass The ray must hit the surface at angle less than 90 degrees Image Fonna onbya Converging Lens Object t K 2F F iF 3 E39 E aEge Image 395 E ue 4 location Ege Eye Thursday September 17 2015 Focusing Light with a Converging Lens Refraction can cause parallel light rays to converge to a focus and form an image Image formation The focal plane is where light from different directions comes into focus The image behind a single convex lens is actually upsidedown Focusing Light Recording an Image Digital cameras detect light and record images with an electronic device called Charge Coupled Device CCD A camera focuses light like an eye and captures the image with a detector The CCD detectors in digital cameras are similar to those used in modern telescopes What are the two basics designs of telescopes 1 Reflecting telescope focuses light with mirrors The curved concave mirror reflects light and forms and image A reflecting telescope has a primary mirror and secondary mirror The secondary mirror can flat or curved The primary mirror us curved concave and can be supported from the back in several places so it can maintain the curvature Reflecting telescopes have much greater diameters Modern telescopes are reflectors 7 Eyepiece lens Parallel light rays from star ii i ll quot i I l i Secondary Parabolic mirror primary mirror Thursday September 17 2015 2 Refracting telescope focuses light with lenses The lens bends light by refraction and forms an image Uses a lens instead of a mirror Disadvantages The lens separates light into different colors It focuses light at different distances along the optical axis This is known as chromatic m Refracting telescopes need to be very long with large heavy lenses Heavy lenses can be supported from the outer part only The weight distorts the shape of the lens causing aberrations in the images Light passing through a lens can get absorbed Absorption can be severe at UV and IR To manufacture a lens it is necessary to machine and polish two surfaces making them more expensive The largest refracting telescope is the 40 diameter Alvan Clark telescope at Yerkes Observator Secondary mirror Primary mirror l Light gt y I I39quot39 lt squot e Light 39g39 T Thursday September 17 2015 What are the two most important properties of a telescope 1 Light collecting area Telescopes with larger mirrors or lenses larger diameter have a large collecting area A large collecting area can gather a greater amount of light in a shorter time A telescope with a large collecting area can observe a fainter object A telescope s diameter tells us its lightcollecting area or light gathering power A pi D2 2 AArea DDiameter aperture of the telescope How does the collecting area of a 10 meter telescope compare with that of a 2 meter telescope using the formula above lt s 25 times greater 2 Angular Resolution Angular Size and Separation Angular size of an object depends on two parameters The physical size of the object The distance to the object Angular size Physical Size Distance Read More Precisely 01 Page 10 Angular Size 360 degrees Physical Size 2 Pi x Distance The Resolving Power of a telescope is the minimum angular separation that the telescope can distinguish Angular Resolution Cont d Ultimate limit to resolution comes from interference of light waves within a telescope Larger telescopes are capable of greater resolution The angular resolution is proportional to wavelength lambda and inversely proportional to the diameter D of the lens or mirror Angular Resolution 025 wavelength diameter Thursday September 17 2015 The central bright spot called the Airy disc increases in size when the diameter of a telescope decreases Larger diameter telescopes produce a small Airy disc and have better resolving power There is a limit in the angular resolution that a telescope can achieve It depends on its diameter The limit in the resolution is known as the diffraction limit Aperture of a telescope the diameter of the mirror or lens 10 arc minutes 10 small aperture 1 arc minute 1 5 arc seconds 5 1 arc second 1 large aperture Resolution of the human eye 05 arc minutes 05 The Magnification of a Telescope how many times bigger an object looks through a telescope compared to how it looks with the naked eye The focal length of a telescope Ft is the equivalent distance from the lens or mirror to the plane where the image forms Thursday September 17 2015 The focal length of an eyepiece Fe is the distance between the lens of the eyepiece and the point where the image forms The magnification of a telescope is the ratio M FtFe The magnification is a number it has no limits Magnifications around 250350 are a practical limit for a telescope A CCD Charged Coupled Device The elements sensitive to light are called pixels A CCD has millions of pixels When light strikes a pile it will develop an electric charge proportional to the intensity of light The charge in each pixel is read and stored as an array of numbers How a CCD stores the information and produces an image The array of numbers is sent to a computer monitor screen Imaging Normally the color filters are red green and blue RGB The software that combines the images assigns colors to each of them Astronomical detectors record forms of light that our eyes can t see xrays gamma rays IR UV Color is sometimes used to represent different energies of nonvisible light Highest energy x rays blue Lowest energy x rays red Medium energy x rays green Thursday September 17 2015 Spectroscopy A spectrograph separates the different wavelengths of light before they hit the detector A diffraction grating is used to separate the colors or wavelengths A diffraction grating has a few thousand lines per mm Photometry and Timing 10 A light curve represents a series of brightness measurements game over a period of time This technique can detect variable stars and variability in astronomical objects Application Measuring the variation of brightness from a star allows to detect the transit of an exoplanet in front of a star


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