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Unformatted text preview: Why is the Sky Blue & Sunsets Red? Properties of Thermal Radiation
1. Wein's Law - Hotter objects emit photons with a higher average energy 2. Stefan-Boltzman Law - Hotter objects emit more power per unit area. Reflected sunlight Thermal emission Both Humans and Planets have Double Hump Spectra What are the three basic types of spectra?
Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Continuous Spectrum Spectra of astrophysical objects are usually combinations of these three basic types 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
Molecular Hydrogen Line Emission Cassini Observations UV Line Emissions from Cloud of Hot Gas Trapped by Jupiter's strong magnetic field 600A 1900A Line Emission / Absorption Spectrum of the Sun Absorption Spectrum of a Real Galaxy - messy! What is this object? What is this object? Reflected Sunlight: Continuous spectrum of visible light is like the Sun's except that some of the blue light has been absorbed - object must look red What is this object? What is this object? Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere What is this object? What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere Mars! R Light spreads out with distance from source Brightness decreases proportional to 1/R2 ...
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