The prevailing approach to modern physics, in both algebra- and calculus-based textbooks, includes a considerable amount of historical exposition combined with physical insight. As an example, Einstein’s successful explanation of the photoelectric effect is very often cited as evidence for the particle nature of light. The purpose of this paper is to inform—or remind—the readers that the latter inference can and should be challenged.

In the photoelectric effect, single-wavelength light strikes a metal surface, and electrons are ejected as a photocurrent. By invoking the possibility of point-like and indivisible energy quanta of light, E = ħν, Einstein was able to explain several observations that classical physics could not. Among them: the generated photocurrent increases linearly with the intensity of the light; the photocurrent measured at the cathode decreases as one increases the opposed (retarding) potential, and becomes zero at the stopping potential; and the stopping potential itself, once a...

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