Experimental Study of the Photoelectric Effect

The apparatus. The standard arrangement used to study the photoelectric effect quantitatively is described in this section's first figure: an evacuated tube containing an emitter plate C, on which the light to be studied is made to fall through a quartz window, and a separate collector plate A, connected through a sensitive microammeter and a battery-and-potentiometer arrangement that lets the potential difference between A and C be set to any chosen value, of either polarity. When light of a suitable frequency falls on C, photoelectrons are ejected from its surface; those that reach A are recorded as a photoelectric current on the microammeter.

Effect of intensity, at fixed frequency and fixed (positive) accelerating voltage. Keeping the frequency of the light fixed (above the threshold value for the metal used) and keeping plate A at a fixed positive potential relative to C (so that every emitted electron is pulled across to A), the photoelectric current is found to rise in direct, linear proportion to the intensity of the incident light -- doubling the intensity exactly doubles the current, confirming that intensity controls the NUMBER of photoelectrons emitted per second, and nothing else.

Effect of the applied (collector) potential, at fixed frequency and fixed intensity. Starting from a large positive potential on A (where every emitted electron reaches A, giving a current called the saturation current) and gradually reducing the potential, the current at first stays unchanged at its saturation value, then begins to fall as the potential is made negative -- because only the FASTER-moving electrons now have enough kinetic energy to fight against the retarding (opposing) electric field and still reach A. The current falls to exactly zero at one particular negative potential, called the stopping potential $V_0$ for that frequency and metal: at this potential, even the fastest photoelectrons emitted (those with the maximum kinetic energy $K_{max}$) are just barely turned back before reaching A. This gives a direct, purely electrical way to measure $K_{max}$, since the work the retarding field does on the fastest electron, $eV_0$, must exactly equal that electron's kinetic energy:

Repeating this at a HIGHER intensity but the SAME frequency gives a LARGER saturation current (more electrons per second) but the SAME stopping potential $V_0$ -- direct proof that $K_{max}$ does not depend on intensity, exactly as Lenard found (Section 11.3). …