![]() Using the photon model, the explanation for this is clear. If we vary the frequency of the EM radiation falling on a clean metal surface, we find the following: For a given material, there is a threshold frequency f 0 for the EM radiation below which no electrons are ejected, regardless of intensity.This event, called the photoelectric effect, is strong evidence for the existence of photons. The manner in which the frequency and intensity of the incoming radiation affect the ejected electrons strongly suggests that electromagnetic radiation is quantized. For simplicity, let us consider what happens with monochromatic EM radiation in which all photons have the same energy hf.įigure 21.7 Incident radiation strikes a clean metal surface, ejecting multiple electrons from it. Some of these properties are inconsistent with the idea that EM radiation is a simple wave. All of these properties are consistent with the idea that individual photons of EM radiation are absorbed by individual electrons in a material, with the electron gaining the photon’s energy. The photoelectric effect would be a key to demonstrating Einstein’s brilliance.Ĭonsider the following five properties of the photoelectric effect. Just as with Planck’s blackbody radiation, Einstein’s concept of the photon could take hold in the scientific community only if it could succeed where classical physics failed. These diagrams will help ensure the students understand fundamental concepts before moving to the difficult proofs that follow. ![]() a low frequency flashlight, and a high wavelength flashlight vs. a high intensity flashlight, a high frequency flashlight vs. Have the students draw photons leaving a low intensity flashlight vs. To ensure that they are, one task that you may have them do is to draw a few pictures similar to Figure 21.6. It is important for students to be comfortable with the material to this point before moving forward. In equation form, Einstein found the energy of a photon or photoelectron to be In fact, the EM wave itself is actually composed of tiny quantum packets of energy called photons. It means that the apparently continuous stream of energy in an EM wave is actually not a continuous stream at all. While these characteristics will be explained a bit later in this section, you can already begin to appreciate why Einstein’s idea is very important. Through careful observations of the photoelectric effect, Albert Einstein realized that there were several characteristics that could be explained only if EM radiation is itself quantized. Using tools similar to the light meter in Figure 21.5, it would take a scientist of Albert Einstein’s stature to fully discover the implications of Max Planck’s radical concept. When Max Planck theorized that energy was quantized in a blackbody radiator, it is unlikely that he would have recognized just how revolutionary his idea was. ![]() Urone) Revolutionary Properties of the Photoelectric Effect A retarding voltage between the collector wire and plate can then be adjusted so as to determine the energy of the ejected electrons. Electrons ejected by the light are collected on the collector wire and measured as a current. Figure 21.5 The photoelectric effect can be observed by allowing light to fall on the metal plate in this evacuated tube.
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