It gained this energy from the incident electromagnetic wave. A photoelectron that leaves the surface has kinetic energy K. To understand why this result is unusual from the point of view of classical physics, we first have to analyze the energy of photoelectrons. For any intensity of incident radiation, whether the intensity is high or low, the value of the stopping potential always stays at one value. For the negative potential difference, as the absolute value of the potential difference increases, the value of the photocurrent decreases and becomes zero at the stopping potential. A higher intensity of radiation produces a higher value of photocurrent. Furthering the potential increase beyond this point does not increase the photocurrent at all. For the positive potential difference, the current steadily grows until it reaches a plateau. Typical experimental curves are shown in (Figure), in which the photocurrent is plotted versus the applied potential difference between the electrodes. The intensity of incident radiation and the kinetic energy of photoelectrons The potential difference at which the photocurrent stops flowing is called the stopping potential. The photocurrent gradually dies out and eventually stops flowing completely at some value of this reversed voltage. Suppose that we now reverse the potential difference between the electrodes so that the target material now connects with the positive terminal of a battery, and then we slowly increase the voltage. But when the target material is connected to the negative terminal of a battery and exposed to radiation, a current is registered in this circuit this current is called the photocurrent. When the target material is not exposed to radiation, no current is registered in this circuit because the circuit is broken (note, there is a gap between the electrodes). The electrodes are enclosed in an evacuated glass tube so that photoelectrons do not lose their kinetic energy on collisions with air molecules in the space between electrodes. The potential difference between the electrodes can be increased or decreased, or its polarity can be reversed. Photoelectrons are collected at the cathode, which is kept at a lower potential with respect to the anode. We call this electrode the photoelectrode. The target material serves as the anode, which becomes the emitter of photoelectrons when it is illuminated by monochromatic radiation. The experimental setup to study the photoelectric effect is shown schematically in (Figure). Electrons that are emitted in this process are called photoelectrons. This phenomenon is known as the photoelectric effect. When a metal surface is exposed to a monochromatic electromagnetic wave of sufficiently short wavelength (or equivalently, above a threshold frequency), the incident radiation is absorbed and the exposed surface emits electrons.
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