The discovery that light can diffract or bend around an obstacle made it appear that light is a wave; but then in 1887, it was discovered that when light is shone on metal, electrons are emitted. In a previous lesson you learned that this emission of electrons from a metal caused by light striking the metal is called the photoelectric effect. Recall that the surprising thing about this discovery is that the brightness of the light does not affect the electrons produced, but the frequencies do.
The frequencies of light relate to the energy of the photons of light that are hitting the metal. The equation to determine the energy is:
\(E = h\nu\ \ \)
If the frequency is higher, then the photon has more energy. This energy will then cause electrons to move faster and be emitted by the metal. So, the more energy the photon has, the more energy the emitted electron will have.
In 1905, Albert Einstein proposed an explanation for the photoelectric effect. Einstein explained the photoelectric effect by assuming that light behaves as if it were a stream of small particles called photons.
The photoelectric effect can be used to explain solar cells which are used in many situations, such as in landscape lights or in large solar panels.
Solar cells work by collecting energy from sunlight. They are made up of photovoltaic cells, which use a process similar to the photoelectric effect. The cells contain two semiconducting plates made up of an element such as silicon. This material conducts electricity when energy is provided. The semiconductor absorbs light and the energy in the light is transferred to electrons within the elements. During the photoelectric effect, these electrons are ejected from the metal.
These electrons are not ejected like they would be in the photoelectric effect. Instead, they accumulate along the boundary between the two plates and create a voltage. This forms a current when the two plates are connected with a wire. The electrons then travel as an electric current. The electricity can be stored in a battery, or it can be used directly.
Sunlight contains the entire spectrum of radiation, but not all the wavelengths are going to be used by the photovoltaic cell. The wavelength must be short enough to have a high enough frequency and thus high enough energy to impart enough energy to an electron for this process to occur--but not too short or the electron will leave the material and not be used for the electric currents.
When you look at solar panels, they are usually black. This is because they are coated with a nonreflective, light-absorbing material. Coating the solar cells with light-absorbing material increases their efficiency because more light will be absorbed instead of reflected.
Question
The photoelectric effect explains how solar cells work by using the particle nature of light, but what part of the wave nature of light is also used to explain solar cells?
The fact that certain wavelengths of light work better is explained by the photoelectric effect. The fact that solar panels are usually black so they will absorb light is explained by the wave properties of light.
Reflection
How does the photoelectric effect provide evidence for the particle nature of light?
