# ASTR 110 Exam #2

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Understand the concept of the EM spectrum, and similarities & differences types of radiation
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COMPLETE SPECTURM OF LIGHT WAVELENGTHS
- Sunlight passes through prism (rainbow color appears), prism separates light into component colors (light of each color is described by wavelength)
- ROY G BIV (longer to shorter wavelengths)
- Infrared waves: longer wavelength light energy, invisible to humans (can be sensed by thermometer
- Ultraviolet: Beyond blue side of spectrum, special instruments can detect shorter wavelengths light energy
- Longest wavelength (radio waves) to shortest wavelength (gamma rays)
- Visible light: portion of EM spectrum our eyes are sensitive to (small fraction of EM spectrum)
HOT, OPAQUE GASSES, LIQUIDS, & SOLIDS ALL EMIT BLACKBODY SPECTRA
Will absorb all wavelengths & reflect one (the one we see)
- Peak of blackbody spectrum is temp. of object determines the wavelength where spectrum has greatest brightness
- Peak of wavelength in blackbody spectrum tells astronomers TEMP OF STAR
Also, can tell temp by looking at color that's most INTENSE
Continuous Spectrum:
1. Black-and-white photograph of a continuous spectrum is completely white b/c light energy at all wavelengths exposes the film
Bright line or Emission spectrum:
1. Lines have same color as corresponding position in a continuous spectrum
2. Dark areas correspond to low brightness on graph whereas lines correspond to peaks in graph (high brightness)
Absorption Spectrum
1. Shows a white background with a series of black vertical lines corresponding to wavelengths with NO LIGHT ENERGY
o Graph of the brightness vs. wavelength of an absorption spectrum shows familiar blackbody curve interrupted by series of downward spikes associated with absorption lines
o Wavelengths of absorption lines correspond exactly to wavelengths of bright lines produced by hot gasses
o Absorption spectrum results from continuous spectrum created by star having particular wavelengths removed as continuous spectrum of light passes through thin outer atmosphere of the star
o Wavelengths that are removed by gasses in outer atmosphere are exactly those that same gasses would produce if heated and observed directly
Reflecting: largest telescopes, use mirrors instead of lenses, image is not as good as glass lens, mirrors are lighter, easier to make, easier to hold in place, and less expensive
- Newtonian reflector used curved mirror in back of telescope to focus light onto a second, smaller mirror near the front telescope, the secondary mirror aims light to a hole in the side of the tube where the eyepiece is located
EXAMPLE: SALT 11 m (South African large telescope)

Refracting: uses only glass lens for magnification, largest lens is at end of telescope closest to object being viewed and it's called objective lens. Second lens is what astronomer looks through and it's the eyepiece lens
- Give clearest and most crisp images
Three main concerns
1. b/c different colors of light pass through glass lenses differently, stars viewed through a refractor can have circular rainbows around them
- Adding a special coating on objective lens can reduce this chromatic aberration effect
2. Weight- hard to hold and heavy
3. Expense- big glass lenses have no flaws, difficult to make and expensive
EXAMPLE: Chabot Space & Science Center in Oakland Cali (20-inch telescope)

Similarities: both concentrate as much light as possible & allow you to see things far
Differences: Reflector- easily make w/ large radius thus making better solution
Refractor- uses a lens as an objective to form an image