ACT Science Question of the Day

A scientist observed that when low energy types of electromagnetic radiation (light), such as visible light or ultraviolet light, were shone onto a conductive copper wire incorporated into a circuit with a light bulb as shown in Figure 1, the light bulb did not come on. However, when electromagnetic radiation of a higher energy, such as x-rays or gamma rays, was shone onto the wire, the light bulb did come on, indicating a current (electricity) was produced in the wire. The scientist hypothesized that this effect was due to the fact that electromagnetic radiation of a certain energy was able to overcome the attraction between electrons and protons in the individual atoms of copper in the wire, and liberate an electron to produce the electric current. He called this the photoelectric effect, and did the following experiments to further study this effect.

Figure 1

Experiment 1:

The scientist put a sample of copper in an instrument that can measure the energies of free-moving electrons. He shone a light source capable of producing specific energies of electromagnetic radiation on the metal and slowly increased the energy of the light. The instrument was used to measure the kinetic energies (energy due to motion) of any electrons that were liberated from the copper sample. The scientist recorded the energy of light used in frequency, where a low frequency corresponds to a low energy electromagnetic radiation, and a high frequency corresponds to a high frequency of electromagnetic radiation. He saw that no electrons could be detected at low frequencies, and that after a specific frequency he called the threshold frequency, the kinetic energy of liberated electrons increased linearly with increasing frequency of incident light as shown in Figure 2 below. The data was fitted with a line along which the data falls; the equation describing this line is shown in Figure 2.

Figure 2

Experiment 2:

The scientist removed the light bulb from the circuit shown in Figure 1 and replaced it with an instrument that can measure electrical charge. He then directed his x-ray source at the wire and slowly increased the intensity of the X-ray light while the frequency was held constant at . He monitored the current produced in the wire as the intensity of light increased and recorded the results in Figure 3 below. Intensity of light is measured in photons (a discrete particle, the fundamental unit of light) and charge in C_oulombs_, and like in Experiment 1, a linear equation is fitted to the data and shown in Figure 3.

Figure 3

Figure 2 shows that the threshold frequency, in Hertz, for copper is most nearly: