1. I Early Experiments anode, and measured using an ammeter as shown in Fig. l.I. The photocathode and the anode are encased in a glass envelope from which the air has been evacuated. The potential difference between the photocathode and the anode is variable as shown and may be either positive or negative. Because the ejected electrons acquire kinetic nergy, the anode voltage VA, if sufficiently negative, can repel them and prevent them from being collected and detected. Several modes of data acquisition are employed, but one of the most striking is a plot of VA versus IA at fixed intensity of the light /. As seen in the hypothetical data in Fig. 1. Ib for three different intensities, the anode current saturates at sufficiently high values of VA, but the value of the stopping voltage VA =-Vs at which the electrons are turned around is independent of the intensity. This shows unequivo- cally that the electron kinetic energy is not determined by the intensity of the light. Moreover, experiments performed with different frequencies show that the value of Vs changes with both the frequency of the light and the material out of which the photocathode is constructed Einstein explained these data in terms of quanta of light called photons. These photons each carry an amount of energy in accord with Equation 1. 1. Thus, the kinetic energy imparted to each electron(having charge of magnitude e)depends upon the energy per photon, not 1, the number of photons per second falling upon the photocathode. Einstein wrote a simple relation between the photon energy hv, the electron kinetic energy K E, and the stopping voltage Vs KE=hv-evs (14) Equation 1. 4 tells us that the kinetic energy of the ejected electron is equal the photon energy hv minus the energy required to liberate the electron from the photo- thode. This amount of energy, called the work function W=evs, differs for each different photocathode material. Equation 1. 4 is usually written in the form KE=hv-w and is known as the Einstein relation It is not our goal here to study the photoelectric effect in detail. We wish to note that Einstein,s explanation clearly showed that light exhibited particle characteris- tics. While the wave properties of light had been known for centuries before the photoelectric effect, its explanation in terms of particles was revolutionary 1.1.2 The franck-Hertz Experiment The Franck-Hertz experiments provided early evidence of the quantization of atomic energy levels. They demonstrated that the amount of energy that could stored in an atom was not arbitrary. Rather, these energies come in discrete incre- ments. Moreover the increments were different for different atoms. For their work first reported in 1914, James Franck and Gustav Ludwig Hertz shared the 1921.1 Early Experiments 3 anode, and measured using an ammeter as shown in Fig. 1.1. The photocathode and the anode are encased in a glass envelope from which the air has been evacuated. The potential difference between the photocathode and the anode is variable as shown and may be either positive or negative. Because the ejected electrons acquire kinetic energy, the anode voltage VA, if sufficiently negative, can repel them and prevent them from being collected and detected. Several modes of data acquisition are employed, but one of the most striking is a plot of VA versus IA at fixed intensity of the light I. As seen in the hypothetical data in Fig. 1.1b for three different intensities, the anode current saturates at sufficiently high values of VA, but the value of the stopping voltage VA = −VS at which the electrons are turned around is independent of the intensity. This shows unequivo￾cally that the electron kinetic energy is not determined by the intensity of the light. Moreover, experiments performed with different frequencies show that the value of VS changes with both the frequency of the light and the material out of which the photocathode is constructed. Einstein explained these data in terms of quanta of light called photons. These photons each carry an amount of energy in accord with Equation 1.1. Thus, the kinetic energy imparted to each electron (having charge of magnitude e) depends upon the energy per photon, not I, the number of photons per second falling upon the photocathode. Einstein wrote a simple relation between the photon energy hν, the electron kinetic energy K E, and the stopping voltage VS K E = hν − eVS (1.4) Equation 1.4 tells us that the kinetic energy of the ejected electron is equal the photon energy hν minus the energy required to liberate the electron from the photo￾cathode. This amount of energy, called the work function W = eVS, differs for each different photocathode material. Equation 1.4 is usually written in the form K E = hν − W (1.5) and is known as the Einstein relation. It is not our goal here to study the photoelectric effect in detail. We wish to note that Einstein’s explanation clearly showed that light exhibited particle characteris￾tics. While the wave properties of light had been known for centuries before the photoelectric effect, its explanation in terms of particles was revolutionary. 1.1.2 The Franck–Hertz Experiment The Franck–Hertz experiments provided early evidence of the quantization of atomic energy levels. They demonstrated that the amount of energy that could be stored in an atom was not arbitrary. Rather, these energies come in discrete incre￾ments. Moreover, the increments were different for different atoms. For their work, first reported in 1914, James Franck and Gustav Ludwig Hertz shared the 1925