Quantum Physics

906 Chapter 28 Atomic PhysicsAPPLICATIONDiscovery of Heliumabsorbing vapor, dark lines will appear at the visible wavelengths 656.3 nm,486.1 nm, 434.1 nm, and 410.2 nm, as shown in Figures 28.3b and 28.4.The absorption spectrum of an element has many practical applications. For example,the continuous spectrum of radiation emitted by the Sun must passthrough the cooler gases of the solar atmosphere before reaching the Earth. Thevarious absorption lines observed in the solar spectrum have been used to identifyelements in the solar atmosphere, including one that was previously unknown.When the solar spectrum was first being studied, some lines were found that didn’tcorrespond to any known element. A new element had been discovered! Becausethe Greek word for Sun is helios, the new element was named helium. It was lateridentified in underground gases on Earth. Scientists are able to examine the lightfrom stars other than our Sun in this way, but elements other than those presenton Earth have never been detected.Applying Physics 28.1Thermal or Spectral?On observing a yellow candle flame, your laboratorypartner claims that the light from the flame originatesfrom excited sodium atoms in the flame. You disagree,stating that because the candle flame is hot, the radiationmust be thermal in origin. Before the disagreementleads to fisticuffs, how could you determine whois correct?Explanation A simple determination could be madeby observing the light from the candle flame througha spectrometer, which is a slit and diffraction gratingcombination discussed in Chapter 25. If the spectrumof the light is continuous, then it’s probably thermalin origin. If the spectrum shows discrete lines, it’satomic in origin. The results of the experiment showthat the light is indeed thermal in origin and originatesfrom random molecular motion in the candleflame.Applying Physics 28.2AurorasAt extreme northern latitudes, the aurora borealisprovides a beautiful and colorful display in thenight sky. A similar display occurs near thesouthern polar region and is called the auroraaustralis. What’s the origin of the various colorsseen in the auroras?Explanation The aurora is due to high speed particlesinteracting with the Earth’s magnetic field andentering the atmosphere. When these particles collidewith molecules in the atmosphere, they excite themolecules in a way similar to the voltage in thespectrum tubes discussed earlier in this section. Inresponse, the molecules emit colors of light accordingto the characteristic spectrum of their atomicconstituents. For our atmosphere, the primaryconstituents are nitrogen and oxygen, which providethe red, blue, and green colors of the aurora.+ er– em eFigure 28.5 Diagram representingBohr’s model of the hydrogen atom.The orbiting electron is allowed onlyin specific orbits of discrete radii.Fv28.3 THE BOHR THEORY OF HYDROGENAt the beginning of the 20th century, scientists were perplexed by the failure ofclassical physics to explain the characteristics of spectra. Why did atoms of a givenelement emit only certain lines? Further, why did the atoms absorb only those wavelengthsthat they emitted? In 1913 Bohr provided an explanation of atomic spectrathat includes some features of the currently accepted theory. Using the simplestatom, hydrogen, Bohr developed a model of what he thought must be the atom’sstructure in an attempt to explain why the atom was stable. His model of the hydrogenatom contains some classical features, as well as some revolutionary postulatesthat could not be justified within the framework of classical physics. The basic assumptionsof the Bohr theory as it applies to the hydrogen atom are as follows:1. The electron moves in circular orbits about the proton under the influence ofthe Coulomb force of attraction, as in Figure 28.5. The Coulomb force producesthe electron’s centripetal acceleration.