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5.4: Photoelectric Effect

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      Image of a solar sail
    Figure \(\PageIndex{1}\) (Credit: NASA/Marshall Space Flight Center; Source: Commons Wikimedia, Solar Sail Tests(opens in new window) []; License: Public domain)

    Are you a fan of science fiction?

    In 1950s science fiction stories, one space travel theme was the use of solar sails for propulsion. The idea was that the photon pressure from the sun would push the sail (like wind sails) and move the spacecraft. What once was science fiction is now reality, as solar sails are being developed and tested for modern space travel.

    Photoelectric Effect and the Particle Nature of Light

    In 1905, Albert Einstein (1879-1955) proposed that light be described as quanta of energy that behave as particles. A photon is a particle of electromagnetic radiation that has zero mass and carries a quantum of energy. The energy of photons of light is quantized according to the \(E = h \nu\) equation. For many years light had been described using only wave concepts, and scientists trained in classical physics found this wave-particle duality of light to be a difficult idea to accept. A key concept that was explained by Einstein using light's particle nature was called the photoelectric effect.

    The photoelectric effect is a phenomenon that occurs when light shone onto a metal surface causes the ejection of electrons from that metal. It was observed that only certain frequencies of light are able to cause the ejection of electrons. If the frequency of the incident light was too low (red light, for example), then no electrons were ejected, even if the intensity of the light was very high or it was shone onto the surface for a long time. If the frequency of the light was higher (green light, for example), then electrons were able to be ejected from the metal surface even if the intensity was very low or it was shone for only a short time. This minimum frequency needed to cause electron ejection is referred to as the threshold frequency.

    Classical physics was unable to explain the photoelectric effect. If classical physics was applied to this situation, the electron in the metal could eventually collect enough energy to be ejected from the surface even if the incoming light was of low frequency. Einstein used the particle theory of light to explain the photoelectric effect as shown in the figure below.

    Figure \(\PageIndex{2}\): Low frequency light (red) is unable to cause ejection of electrons from the metal surface. At or above the threshold frequency (green) electrons are ejected. Even higher frequency incoming light (blue) causes ejection of the same number of electrons, but with greater speed. (Credit: Raymond Chou; Source: CK-12 Foundation; License: CC BY-NC-SA 3.0(opens in new window))

    Consider the \(E = h \nu\) equation. The \(E\) is the minimum energy that is required in order for the metal's electron to be ejected. If the incoming light's frequency, \(\nu\), is below the threshold frequency, there will never be enough energy to cause electrons to be ejected. If the frequency is equal to or higher than the threshold frequency, electrons will be ejected. As the frequency increases beyond the threshold, the ejected electrons simply move faster. An increase in the intensity of incoming light that is above the threshold frequency causes the number of electrons that are ejected to increase, but they do not travel any faster. The photoelectric effect is applied in devices called photoelectric cells, which are commonly found in everyday items (such as a calculator) that use the energy of light to generate electricity.

    Scientific solar powered calculator.
    Figure \(\PageIndex{3}\): Photoelectric cells convert light energy into electrical energy, which powers this calculator. (Credit: Sergei Frolov; Source: Commons Wikimedia, File:FX-77.JPG(opens in new window) []; License: Public Domain)



    Launch the PLIX interactive below to determine which type of colored light has enough energy (eV) to eject an electron from the metal plate:


    • Light has properties of both a wave and a particle.
    • The photoelectric effect is produced by light striking a metal and dislodging electrons from the surface of the metal.


    1. What are the properties of a photon?
    2. What does the photoelectric effect show about the properties of light?
    3. How does the frequency of light affect the release of photons?

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