It was not the discovery itself that dealt the fatal blow to classical physics, but the specific characteristics of the photoelectric effect found around 1900 by one of Hertz’s students, Phillip Lenard. Lenard built a glass tube, shown in FIGURE 38.1, with two facing electrodes and a window. After removing the air from the tube, he allowed light to shine on the cathode.

FIGURE 38.1 Lenard’s experimental device to study the photoelectric effect.

Lenard found a counterclockwise current (clockwise flow of electrons) through the ammeter whenever ultraviolet light was shining on the cathode. There are no junctions in this circuit, so the current must be the same all the way around the loop. The current in the space between the cathode and the anode consists of electrons moving freely through the evacuated space between the electrodes (i.e., not inside a wire) at the same rate (same number of electrons per second) as the current in the wire. There is no current if the electrodes are in the dark, so electrons don’t spontaneously leap off the cathode. Instead, the light causes electrons to be ejected from the cathode at a steady rate.

Lenard established an adjustable potential difference $\text{Δ}V$ between the two electrodes. He then studied how the current $I$ varied as the potential difference and the light’s frequency and intensity were changed. Lenard made the following observations:

1. The current $I$ is directly proportional to the light intensity. If the light intensity is doubled, the current also doubles.

2. The current appears without delay when the light is applied. To Lenard, this meant within the $\approx 0.1\text{s}$ with which his equipment could respond. Later experiments showed that the current begins in less than 1 ns.

3. Photoelectrons are emitted only if the light frequency $f=c\text{/}\lambda$ exceeds a threshold frequency $f_0$. This is shown in the graph of FIGURE 38.2. That is, the photoelectron emission is observed for higher-frequency electromagnetic waves (shorter wavelengths) but not for lower-frequency waves (longer wavelengths).

   FIGURE 38.2 The photoelectric current as a function of the light frequency $f$ for light of constant intensity.

   

   FIGURE 38.2 Full Alternative Text

4. The value of the threshold frequency $f_0$ depends on the type of metal from which the cathode is made.

5. If the potential difference $\text{Δ}V$ is more than about 1 V positive (anode positive with respect to the cathode), the current does not change as $\text{Δ}V$ is increased. If $\text{Δ}V$ is made negative (anode negative with respect to the cathode), by reversing the battery, the current decreases until, at some voltage $\text{Δ}V=-V_{\text{stop}}$ the current reaches zero. The value of $V_{\text{stop}}$ is called the stopping potential. This behavior is shown in FIGURE 38.3.

   FIGURE 38.3 The photoelectric current as a function of the battery potential for light with frequency $f>f_0$.

   

   FIGURE 38.3 Full Alternative Text

6. The value of $V_{\text{stop}}$ is the same for both weak light and intense light. A more intense light causes a larger current, as Figure 38.3 shows, but in both cases the current ceases when $\text{Δ}V=-V_{\text{stop}}$.