Evidence for Wave-Particle Duality

FIGURE 4.14 In a photoelectric cell (photocell), light strikes a metal surface in an evacuated space and ejects electrons. The electrons are attracted to a positively charged collector, and a current flows through the cell. 

FIGURE 4.15 Frequency and intensity dependence of the photoelectric effect. Only light above the threshold frequency can eject photoelectrons from the surface. Once the frequency threshold has been passed, the total current of photoelectrons emitted depends on the intensity of the light, not on its frequency. 

FIGURE 4.16 (a) Two key aspects of the photoelectric effect. Blue light is effective in ejecting electrons from the surface of this metal, but red light is not. (b) The maximum kinetic energy of the ejected electrons varies linearly with the frequency of light used. 

The current in a photocell depends on the potential between cathode and collector. 

Light with a wavelength of 400 nm strikes the surface of cesium in a photocell, and the maximum kinetic energy of the electrons ejected is 1.54 X 10 J. Calculate the work function of cesium and the longest wavelength of light that is capable of ejecting electrons from that metal. 

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Criterion 4 Role of Experiments by G. P. Thomson. Thomson was also independently working to provide experimental evidence for wave-particle duality and reported his results to Nature about two months after Davisson (Medicus 1974). This shows how various research groups may work on the same problem and arrive at the same conclusion using different experimental techniques. Furthermore, it illustrates that there is no one way of doing science. For this criterion to be met, it is important for the textbooks to describe the following aspects ... 

Experiments are not only difficult to perform but also lead to alternative interpretations and consequently controversies (Criterion 3). Davisson-Germer had considerable difficulty in understanding their experimental data before it could be accepted as evidence for wave-particle duality. Even leading scientists (Planck, Bohr) were reluctant to accept the wave-particle duality as it conflicted with some of the predictions based on the previous dominant paradigm, namely the wave theory of light. 

What experimental evidence supports the quantum theory of light Explain the wave-particle duality of all matter. For what size particles must one consider both the wave and the particle properties ... 

The relationship between theoretical ideas and experimental evidence is important (Criterion 1). As there is no one way of doing science, at times experimental observations can precede the theoretical formulations and similarly the inverse relationship as demonstrated by the wave-particle duality, is also possible. However, it is important to note that experimental data do not unambiguously provide evidence for a particular framework and hence the importance of the role played by the scientific community that often leads to conflicts and controversies. 

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