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Unformatted text preview: Radiation Laws and Spectra Last time The nature of light Interaction of light and matter EM spectrum Properties of matter What is the electromagnetic spectrum? What is the structure of matter?
Electron Cloud Atom Nucleus Energy Level Transitions The only allowed changes in energy are those corresponding to a transition between energy levels Continuous Spectrum Not Allowed Allowed The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption Emission Line Spectrum Absorption Line Spectrum A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum Chemical Fingerprints Each type of atom has a unique set of energy levels Each transition corresponds to a unique photon energy, frequency, and wavelength Chemical Fingerprints Downward transitions produce a unique pattern of emission lines Energy levels of Hydrogen Chemical Fingerprints Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths Chemical Fingerprints Each type of atom has a unique spectral fingerprint Chemical Fingerprints Energy Levels of Molecules Observing the fingerprints in a spectrum tells us which kinds of atoms are present Molecules have additional energy levels because they can vibrate and rotate. Energy Levels of Molecules Example: Solar Spectrum Spectrum of Molecular Hydrogen The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated. Many of these molecular transitions are in the infrared part of the spectrum. Thought Question Which letter(s) labels absorption lines? Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E A B C D E Thought Question Which letter(s) labels emission lines? What are the three basic types of spectra?
Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum A B C D E Spectra of astrophysical objects are usually combinations of these three basic types. How does light tell us the temperatures of planets and stars? Thermal Radiation Dense warm objects emit thermal radiation, including stars, planets, you... A thermal body's spectrum depends on only one property: its temperature. Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies per unit area. 2. Hotter objects emit photons with a higher average energy. Wien's Law
The Wavelength at which a thermal emitter is brightest is shorter for higher temperatures: !max 2 " 10 6 = T Where the wavelength is in nm and T is in degrees K. Degrees K = degrees C + 273. Stefan-Boltzmann Law
The energy emitted by a thermal body depends on temperature raised to the fourth power: Thought Question Which is hotter?
a) A blue star. b) A red star. c) A planet that emits only infrared light. E = 5 ! 10 "5 ! T 4
E has the units of erg/s cm2, and T is in degrees K. How does light tell us the speed of a distant object? Measuring the Shift
Stationary Moving Away Away Faster Moving Toward The Doppler Effect Toward Faster We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines. ...
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This note was uploaded on 03/26/2008 for the course ASTRO 150 taught by Professor Guillermogonzalez during the Fall '07 term at Iowa State.
- Fall '07