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Clicker Question: The energy of a photon is proportional to its: A: period B: amplitude C: frequency D: wavelength Clicker Question: A star much colder than the sun would appear: A: red B: yellow C: blue D: smaller E: larger Betelgeuse Rigel Betelgeuse The total energy radiated from entire surface every second is called the luminosity. Thus Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2) For a sphere, area of surface is 4pR2, where R is the sphere's radius. The "Inverse-Square" Law Applies to Radiation Each square gets 1/4 of the light Each square gets 1/9 of the light apparent brightness a 1 D2 D is the distance between source and observer. The Doppler Effect Applies to all kinds of waves, not just radiation. at rest velocity v1 velocity v2 velocity v1 velocity v1 velocity v3 you encounter more wavecrests per second => higher frequency! fewer wavecrests per second => lower frequency! Doppler Effect Demo: buzzer on a moving arm Demo: The Doppler Ball The frequency or wavelength of a wave depends on the relative motion of the source and the observer. Clicker Question: If a star is moving rapidly towards Earth then its spectrum will be: A: the same as if it were at rest B: shifted to the blue C: shifted to the red D: much brighter than if it were at rest E: much fainter than if it were at rest Example: Blackbody - the microwave background Spectroscopy and Atoms How do you make a spectrum? For light, separate white light into its colors using a glass prism or "diffraction grating". For radiation in general, spread out the radiation by wavelength (e.g car radio, satellite TV receiver). How we know these things: - Physical states of stars, gas clouds, e.g. temperature, density, pressure. - Chemical make-up of stars, galaxies, gas clouds - Ages of stars and galaxies - Masses of stars, clouds, galaxies, extrasolar planets, rotation of galaxies, expansion of universe, acceleration of universe. All rely on taking and understanding spectra Types of Spectra 1. "Continuous" spectrum - radiation over a broad range of wavelengths (light: bright at every color). 2. "Emission line" spectrum - bright at specific wavelengths only. 3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines. Kirchhoff's Laws 1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum. 2. A transparent hot gas produces an emission line spectrum. 3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum. The pattern of emission (or absorption) lines is a fingerprint of the element in the gas (such as hydrogen, neon, etc.) For a given element, emission and absorption lines occur at the same wavlengths. Sodium emission and absorption spectra Demo - Spectra Demo - Spectrum of the sun Spectrum of Helium (He) Gas Discovered in 1868 by Pierre Jannsen during a solar eclipse Subsequently seen and named by Norman Lockyer Example: spectra - comet Hyakutake