Sec 12.4.2 - Photons 415 perhaps even zero frequency f = 0...

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Photons 415 12.4.2 Absorption of Light by Metals: The Photoelectric Effect Typically, most of the visible light that strikes a shiny, polished piece of metal is refl ected from the surface. Think of the reflection you can see of your face in the sur- face of a shiny stainless-steel cooking pan. Although most of the light is refl ected, a small amount of the light energy striking the metal can be absorbed by electrons near the surface of the metal. If the frequency of the light is high enough (in the UV), a curious thing occurs, as illustrated by the energy-level diagram in Figure 12.7 . An electron can gain enough energy by absorbing a photon that it can escape from the metal altogether. The stick person moving off to the right illustrates this in the fi gure. The electron is ejected from the metal’s surface into the air. The minimum energy needed to eject the electron from the metal is called the threshold energy and is denoted by E THR . The minimum fre- quency that light must have to cause an electron to be ejected is therefore: f E h THR = That is, the light’s frequency f must be greater than E THR / h to eject an electron. As an example, the metal zinc has threshold energy equal to 0.69 aJ. This means that to eject an electron, light must have a frequency greater than f E h h THR = = = × × 0 69 0 69 10 6 6 10 18 34 . . / . aJ J photon J photon Hz = × sec/ . 1 04 10 15 The ejection of electrons from metal by light is called the photoelectric effect , and was fi rst studied in detail by Phillip Lenard, a Hungarian-German physicist, in 1902. A few years later, in 1905, Albert Einstein offered the explanation that we used in the above paragraphs to explain the effect. For this, he was awarded the Nobel Prize. perhaps even zero frequency ( f = 0). According to Quantum Principle (v), such low-fre- quency electromagnetic oscillations cannot excite an electron to jump a high-energy gap. With f = 0, we certainly cannot satisfy the requirement that h f is greater than E GAP , which is needed for the electron to make an energy jump. This explains why a constant voltage cannot accelerate an electron in an insulator. FIGURE 12.7 Photoelectric effect: A photon with high enough energy h f is absorbed by a metal, causing an electron in the metal to make an upward quantum jump to a sufficiently high energy that it escapes the metal. E L E THR Jump E H - E L = hf hf E H Energy increasing TAF-K10173-08-1107-012.indd 415 TAF-K10173-08-1107-012.indd 415 4/27/09 6:58:11 AM 4/27/09 6:58:11 AM
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416 The Silicon Web: Physics for the Internet Age Lenard made several key observations, which allowed Einstein to put forth the explanation that we have discussed: 1. If the light’s frequency was below a certain threshold value (which depended on the type of metal used), then no electrons were ejected from the metal (see Figure 12.8a) . In this case, increasing the light’s power had no effect—still no electrons were ejected.
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