studying for the exam relevant chapters: e, 1, 2 & 3 to prepare for the exam it is helpful to...
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Studying for the Exam
• Relevant chapters: E, 1, 2 & 3
• To prepare for the exam it is helpful to …– review readings– review lecture notes online (esp. concept
questions)– revisit homework– look over the activities
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Studying for the Exam
• Filter out the relevant information, don’t focus on details
• Properties of relevant information:– Information appears repeatedly in course
materials (readings, slides, homework,…)– It is not an isolated fact, but can be “reasoned
out”– It is an important concept (e.g. daily &
monthly motion, scientific method)
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Exam Questions
• About 30 multiple choice questions
• A few short answer problems
• Types of questions NOT on the exam: – What’s Galileo’s birth year?– What is the frequency of yellow light?– What is the distance of the Earth to the Sun?– What is the mathematical formula for the
Hydrogen energy levels?
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Exam Questions
• Types of questions that could be on the exam: – Why isn’t there a lunar eclipse every full moon?– It is noon in Westerville. Is it earlier/ later/different
day/different season in Paris, France? – What is the difference between a sidereal and a solar
day? – How high above the horizon is the polar star at noon if
you are at 23 degrees northern latitude? – Given the wavelength of yellow light, what is its
frequency?
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Light hits Matter: Refraction• Light travels at different speeds in vacuum, air,
and other substances• When light hits the material at an angle, part of it
slows down while the rest continues at the original speed – results in a change of direction– Different colors bend different amounts – prism,
rainbow
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Application for Refraction
• Lenses use refraction to focus light to a single spot
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Light hits Matter (II): Reflection• Light that hits a mirror is
reflected at the same angle it was incident from
• Proper design of a mirror (the shape of a parabola) can focus all rays incident on the mirror to a single place
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Application for Reflection
• Curved mirrors use reflection to focus light to a single spot
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Telescopes
• From Galileo to Hubble
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Telescopes• Light collectors• Two types:
– Reflectors (Mirrors)– Refractors
(Lenses)• Magnification:
– ratio of focal lengths of objective and eyepiece
– M = fobj/feye
– Example: 2000mm telescope with 40mm eyepiece: 50x
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Reflecting Telescopes
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Problems with Refractors
• Different colors (wavelengths) bent by different amounts – chromatic aberration
• Other forms of aberration
• Deform under their own weight
• Absorption of light
• Have two surfaces that must be optically perfect
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Telescope Size• A larger telescope gathers more light (more
collecting area)
• Angular resolution is limited by diffraction of light waves; this also improves with larger telescope size
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Resolving Power of Telescopes
Andromeda Galaxy
Telescope 1 Telescope 2 of double size
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Resolving Power of Telescopes (II)
Andromeda Galaxy
Resolution:
(a) 10’
(b) 1’
(c) 5”
(d) 1”
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Magnification
• The magnification of a telescope can easily be changed by plugging in a different eyepiece with a different focal length
• M= focal length of main lense or mirror
focal length of eyepiece
Example: F= 2000mm, f = 40 mm M= 50
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Atmospheric Limitations
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Atmospheric Limitations
Radio Window
Optical Window
IR Window
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Largest Earth-Based Telescopes• Hobby-Eberly Telescope,
Davis Mountains, TX– 11 m diameter– Cannot see all parts of the sky
• Keck I and II, Mauna Kea, HI– 36 1.8 m hexagonal mirrors;
equivalent to 10 m– Above most of atmosphere
(almost 14,000 ft ASL)– Operating since 1993
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Other Techniques
• Put telescopes on satellites– Hubble Space
Telescope: 2.4 m, since 1990
• Use computers to correct optics during light gathering: adaptive and active optics
• Interferometry• Radio astronomy
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Other Wavelengths• Must be carried out on satellites (or rockets,
balloons, etc.) due to strong absorption in the atmosphere
• Infrared astronomy• High-energy (UV, X-ray, gamma-ray) astronomy
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Full-Spectrum Coverage
Each region of the electromagnetic spectrum gives us valuable information about the universe
(only these frequency bands can be observed with ground-based telescopes)
• Radio
• Infrared
• Visible
• X-Ray
• Gamma-ray