2Light Charges interact via electric and magnetic forces Light is a repetitive disturbance in these forces!Electromagnetic waveA form of energyDepending on conditions, light can also act like a particleA photon
12Spectra We will consider three types of spectra: Emission Absorption Continuous
13Spectra Emission and Absorption spectra: An atom can both emit and absorb lightWe consider a gas that is not denseIf we don’t, the atoms interact and alter the energy levelsThis ruins the unique spectral fingerprint of the individual atoms
14Emission SpectraIf a gas has enough microscopic energy (high enough temperature):Collisions between atoms will transfer energy to electronsElectrons then drop to a lower energy level, emitting a photon
18Absorption Spectra White light is shining through a cold gas cloud. White light contains all colors of the spectrum.The atoms in the gas cloud absorb photons with energies corresponding to differences in atomic energy levelsThese colors are therefore removed from the spectrum
22Continuous SpectraThe spectrum of a hot dense object has a bump-shaped graphThe graph shows the brightness of each color (wavelength)What determines the exact shape?
23Continuous SpectraThe amount of energy emitted by such an object is given by the Stefan-Boltzmann law:Brightness = sT4The wavelength at which the peak occurs is given by Wien’s LawHigher temperature object peaks at shorter wavelengths
24Temperature Estimation So we can use a glowing object’s color to estimate its temperatureDoes this apply to the lava? How about an orange shirt?Why or why not?
26Spectrum of Mars Is there evidence of continuous spectra? Are there emission lines?Absorption lines?
27Spectrum of Mars The boxed region shows reflected sunlight What color is Mars?The Sun is a source of a continuous spectrum
28Spectrum of Mars What causes the lower-energy bump? Does Mars glow? Do we see its glow?
29Spectrum of MarsThis is the signature of carbon dioxide.
30Spectrum of Mars Emission lines These come from hot gas in the atmosphere
31Doppler EffectBecause of the wave nature of light, its frequency or wavelength change if the source moves toward or away from an observer.
32Doppler Effect An object’s spectrum blueshifts if it moves toward us An object’s spectrum redshifts if it moves away from us
33Doppler EffectBased on an object’s redshift or blueshift, we can ascertain how fast it is moving toward or away from usWe know where the lines should be when the object is at rest, so we can easily measure the shift