Quantum Physics

922 Chapter 28 Atomic PhysicsAtom inexcited stateAtom inground stateAtom inexcited stateAtom inground stateE 2E 2E 2E 2∆Ehf = ∆Ehf = ∆E∆EhfE 1 E 1BeforeAfterACTIVE FIGURE 28.18Diagram representing the process ofspontaneous emission of a photon by anatom that is initially in the excitedstate E 2 . When the electron falls tothe ground state, the atom emits aphoton of energy hf E 2 E 1 .E 1 E 1BeforeAfterACTIVE FIGURE 28.19Diagram representing the process of stimulated emission of a photon by an incoming photon of energyhf. Initially, the atom is in the excited state. The incoming photon stimulates the atom to emit a secondphoton of energy hf E 2 E 1 .hfLog into PhysicsNow atwww.cp7e.com and go to ActiveFigure 28.18 to observe spontaneousemission.Log into PhysicsNow at www.cp7e.com and go to Active Figure 28.19 to observe stimulated emission.This process is known as spontaneous emission. Typically, an atom will remain in anexcited state for only about 10 8 s.A third process that is important in lasers, stimulated emission, was predicted byEinstein in 1917. Suppose an atom is in the excited state E 2 , as in Active Figure28.19, and a photon with energy hf E 2 E 1 is incident on it. The incoming photonincreases the probability that the excited atom will return to the ground stateand thereby emit a second photon having the same energy hf. Note that twoidentical photons result from stimulated emission: the incident photon and theemitted photon. The emitted photon is exactly in phase with the incident photon. Thesephotons can stimulate other atoms to emit photons in a chain of similar processes.The many photons produced in this fashion are the source of the intense, coherent(in-phase) light in a laser.Applying Physics 28.4Streaking MeteoroidsA physics student is watching a meteor shower in theearly morning hours. She notices that the streaks oflight from the meteoroids entering the very highregions of the atmosphere last for as long as 2 or 3seconds before fading. She also notices a lightningstorm off in the distance. The streaks of light from thelightning fade away almost immediately after the flash,certainly in much less than 1 second. Both lightningand meteors cause the air to turn into a plasma becauseof the very high temperatures generated. Thelight is given off when the stripped electrons in theplasma recombine with the ionized atoms. Why wouldthe light last longer for meteors than for lightning?Explanation To answer this question, we examinethe phrase “the streaks of light from the meteoroidsentering the very high regions of the atmosphere.” Inthe very high regions of the atmosphere, the pressure isvery low, so the density is also very low and the atoms ofthe gas are relatively far apart. Low density means thatafter the air is ionized by the passing meteoroid, theprobability of freed electrons finding an ionized atomwith which to recombine is relatively low. As a result, therecombination process occurs over a relatively long time,measured in seconds. Lightning, however, occurs in thelower regions of the atmosphere (the troposphere),where the pressure and density are relatively high. Afterthe ionization by the lightning flash, the electrons andionized atoms are much closer together than in theupper atmosphere. The probability of a recombinationis accordingly much higher, and the time for therecombination to occur is much shorter.28.12 LASERS AND HOLOGRAPHYWe have described how an incident photon can cause atomic transitions either upward(stimulated absorption) or downward (stimulated emission). The twoprocesses are equally probable. When light is incident on a system of atoms, there