The Photoelectric Effect

The science and history behind the phenomenon known as "The Photoelectric Effect"

The photoelectric effect was a groundbreaking discovery in the world of physics in the early 1900’s. It was one of the first applications of quantum mechanics. Many scientists studied and contributed to the understanding of the effect and the connotations to the idea of light it provided. These scientists included Planck, Hertz, Lenard and Einstein. However, among these it was Einstein who pieced all of there work together in 1905.

            The first observations of the photoelectric effect were made by Heinrich Hertz in 1887. Hertz was trying to prove Maxwell’s theories of electromagnetism and set up a circuit that caused sparks to oscillate between two pieces of brass. Hertz noticed that when he shone an ultraviolet light on the brass that the sparks were amplified. Hertz failed to explain this phenomenon and it wasn’t until the discovery of the electron in 1899 by J.J Thompson that it could be explained as the emission of electrons from the metal surface. This emission of electrons from the energized surface is what’s known as the photoelectric effect.

            In 1900 Max Planck was studying the connection between the intensity and frequency of black body radiation. In order to do this Planck suggested that he couldn’t treat the radiation as continuous energy but had to treat it rather as an exertion of equal discrete particles or “quanta” of energy.  This was by what some considered the birth of quantum mechanics.  He discovered that the energy from the radiation was dependent on the frequency and derived the formula E=hf (Figure 2) where “h” (Figure 1) is the proportionality constant. This theory did not convince too many as there had been more evidence to support the wave theory of light.

In 1902 the physicist Philipp Lenard continued with the study of the photoelectric effect. He was looking for the relationship between the intensity of the light and the kinetic energy of the emitted electron. However, he soon discovered that the kinetic energy of the electron did not depend upon the intensity of the light but rather upon the frequency. The amount of electrons emitted depended on the intensity, but the kinetic energy would not increase after a certain point no matter how intense the light.           

Finally, in 1905 it was Albert Einstein who discovered the relationship between the photoelectric effect and Max Planck’s work. He proposed the idea that light could exist as individual particles with a certain quanta of energy called photons. He found that Planck’s formula represented the energy of the photons and that each photon could only emit one electron. Einstein concluded that the kinetic energy of the emitted electron (Figure 3) was equal to the energy of the photon minus the energy required to emit the electron from the object. From this we can see that light needs a certain frequency to eject electrons from the object and even higher for electrons deeper within the object. This theory caused problems with the classic wave theory of light and was disbelieved for some time until accidentally proved by Milikan in 1916.

The photoelectric effect is the phenomenon that when light shines on a metal surface, electrons are emitted from the surface (Figure 4). One photon from light can eject one electron and the kinetic energy of this “photoelectron” depends on the frequency of that light. The intensity of the light will determine the amount of electrons emitted because of more photons but it is only the frequency that affects the energy. All of this however can only be understood by applying the concept of quantum theory of light. In conclusion, we can see that light behaves as both particles and waves; making it safe to say that light is defined as a wave-particle duality.

Figure 1: Planck’s constant (h) = 6.6260755 x 10-34 J∙s

Figure 2: Planck’s formula = E=hf

Figure 3: Energy of photoelectron = hf – W

Figure 4: The Photoelectric Effect

 

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