AS101 Lecture 14

# AS101 Lecture 14 - AS101 Lecture 14 Logistics Homework 6...

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AS101 Lecture 14 Logistics: • Homework 6 due today • Homework 7 available • Do night lab as soon as you can Last Lecture: • Atom • Light Today’s Lecture: • Light and matter • Doppler Effect • Telescopes • Spectrometer. interferometer

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Light and Matter • When light interacts with matter several things can happen – Absorption – Transmission – Reflection
Spectrum • Light intensity at different wavelengths • Different types of spectra (the third guy with 3 laws – Kirchhoff) – Continuous (also called blackbody radiation ) - spectrum without sharp discontinuities (Usually spectra from solids and liquids are only this type. Also get continuous spectrum from gas) – Emission - only from gasses (when observed directly from a source) – Absorption - also only from gasses (seen when light from a source with continuous spectrum is passed through an absorbing material) http://www-gap.dcs.st-and.ac.uk/~history/PictDisplay/Kirchhoff.html

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Continuous spectrum • Continuous spectrum - thermal radiation - radiation coming from opaque objects (blackbody) • Hotter objects emit more total radiation per unit surface area (Stefan’s law) • Hotter objects emit photons with higher average energy (Wein’s law)
Perfect radiator ( also called blackbody ) • Absorbs all incident energy (they produce the greatest amount of energy for a given temperature) • The temperature of a blackbody depends on the total radiant energy incident on it per unit time • It’s spectrum is described by Planck’s law E = hc 3 / 5 (e hc/ kT - 1) • Stars are nearly blackbodies (blackbodies do not have to be black); we could infer their temperature by studying their spectra You don’t need to know this formula

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This is what the formula shows Elena Fullerton: Does the light bulb give off more IR radiation when it is hot or cold?
More on Blackbody radiation • The wavelength of the maximum of each curve ( max ) depends on the temperature as given

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AS101 Lecture 14 - AS101 Lecture 14 Logistics Homework 6...

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