Name: why light ejects electrons from metals
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 26 s
Channel: JoVE

K12 Standards

Pennsylvania State Academic Standards

9-12

Physical Science

Waves and Electromagnetic Radiation

3.2.9-12.X

Photoelectric Effect

When light of a particular wavelength and frequency strikes a metal, electrons are ejected from the metal. This phenomenon is known as the photoelectric effect. 
However, only light above a threshold frequency can eject electrons from the metal. Lower frequency light does not eject electrons, regardless of its intensity. How is this so?
Albert Einstein proposed that light behaves as a stream of particles or tiny packets. One packet &#8212; or quantum &#8212; of light is called a photon.
Each photon has an energy, E, that depends on its frequency, &#957;. The two are related by this equation, where h is Planck&#8217;s constant and has a value of 6.626 &#215; 10&#8722;34 J&#8901;s. Light, and thus photons, of higher frequency have greater energy.
An electron is bound to a metal with a binding energy, phi. This is also known as the work function of the metal, W. When a photon with energy greater than phi strikes the metal, its excess energy is transferred to the electron as kinetic energy and the electron is ejected. Thus, the kinetic energy of the electron is the difference between the energy of the photon and the binding energy of the electron.
To summarize the photoelectric effect, low-frequency, long-wavelength light does not eject any electrons from the metal. Increasing the intensity of low-frequency light only increases the number of photons. Each photon still has the same energy, which is not enough to dislodge any electrons. 
Shorter wavelength light with a frequency higher than the threshold frequency does eject electrons because each photon is higher energy. As the frequency of light increases, so does the energy of the photon, and thus, the kinetic energy of the ejected electron. 
The greater the intensity of light, the more photons strike the metal, and the greater the number of electrons that are ejected.
The photoelectric effect demonstrates the particle behavior of light.

photoelectric effect

When light of a particular wavelength strikes a metal surface, electrons are emitted. This is called the photoelectric effect. The minimum frequency of light that can cause such emission of electrons is called the threshold frequency, which is specific to the metal. Light with a frequency lower than the threshold frequency, even if it is of high intensity, cannot initiate the emission of electrons. However, when the frequency is higher than the threshold value, the number of electrons ejected is directly proportional to the intensity of the beam.
According to classical wave theory, a wave's energy depends on its intensity (which depends on its amplitude), not its frequency. One part of these observations was that the number of electrons ejected within a given time period was seen to increase as the brightness increased. In 1905, Albert Einstein was able to resolve the paradox by incorporating Planck's quantization findings into the discredited particle view of light.
Einstein argued that the quantized energies that Planck had postulated could be applied to the light in the photoelectric effect. The light striking the metal surface should not be viewed as a wave, but should instead be viewed as a stream of particles (later called photons) whose energy depended on their frequency, The amount of energy (E) in a light packet depends on its frequency (&#957;) according to the following equation:
<img alt="Photon energy equation \(E=h\nu\), fundamental physics formula, symbol representation." src="/files/core/CDNSource/lessons/976/976_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
where h is Planck&#8217;s constant.
The photoelectric effect can be described by assuming that the light is quantized. A certain minimum energy is required to overcome the binding energy (&#934;) experienced by an electron. This is also known as the work function (W) of the metal.
Since the electrons in the metal had a certain amount of binding energy keeping them there, the incident light needs to have more energy to free the electrons. Photons of low-frequency light do not contain enough energy to eject electrons from the metal. Even if the metal is exposed to such light for a long time, no emission of electrons is observed. An electron can only be emitted when a photon with energy greater than the work function strikes the metal.
<img alt="photoelectric effect formula: \(h\nu = KE + W\) equation for photon energy and work function." src="/files/core/CDNSource/lessons/976/976_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
The excess energy of the photon is converted into kinetic energy of the emitted electron.
<img alt="Photoelectric effect equation: KE=hv-W, describing energy of emitted electrons, formula depiction." src="/files/core/CDNSource/lessons/976/976_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
Therefore, electrons are ejected when they are hit by photons having sufficient energy (a frequency greater than the threshold). The greater the frequency of incident light, the greater the kinetic energy imparted by the collisions to the escaping electrons. Einstein also argued that the light intensity did not depend on the amplitude of the incoming wave, but instead corresponded to the number of photons striking the surface within a given time period. The number of ejected electrons increases with brightness. The greater the number of incoming photons, the more likely that they will collide with some of the electrons.
The photoelectric effect strongly suggests the particle behavior of light. Einstein won the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect. Although many light phenomena could be explained either in terms of waves or particles, certain phenomena, such as the interference patterns obtained when light passed through a double slit, were completely contrary to a particle view of light, while other phenomena, such as the photoelectric effect, were completely contrary to a wave view of light. Somehow, at a deep fundamental level still not fully understood, light is both wavelike and particle-like. This is known as wave-particle duality.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/6-1-electromagnetic-energy">Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy</a>.

When light of a particular wavelength strikes a metal surface, electrons are emitted. This is called the photoelectric effect. The minimum frequency of ...

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KEY TERMS AND CONCEPTS

why light ejects electrons from metals

Light can eject electrons from a metal surface when it has the right frequency. This process is called the photoelectric effect. Each metal has its own threshold frequency, which is the minimum light frequency needed to free electrons.

Light below the threshold frequency cannot cause emission, even if it is very bright. When the light frequency is above the threshold, electrons are ejected. In that case, the number of electrons released is directly proportional to the intensity, or brightness, of the beam.

Classical wave theory could not explain this result because it links a wave’s energy to its intensity, not its frequency. In 1905, Albert Einstein solved the problem by using Planck’s idea that energy is quantized. He treated light as a stream of particles called photons, and each photon carries energy based on its frequency.

A metal holds onto its electrons with a minimum amount of binding energy. This energy barrier is called the work function, or Φ, of the metal. Low-frequency photons do not have enough energy to overcome it, so no electrons are emitted even after long exposure. If a photon has more energy than the work function, the extra energy becomes kinetic energy of the escaping electron.

Higher-frequency light gives electrons more kinetic energy. Brightness also matters because intensity reflects the number of photons striking the surface in a given time. More photons increase the chance that some will hit electrons and free them.

The photoelectric effect supports the particle behavior of light. Einstein won the Nobel Prize in Physics in 1921 for explaining it. At the same time, other light behavior, such as the interference pattern in a double slit, does not fit a simple particle model. Together, these observations show wave-particle duality, the idea that light has both wavelike and particle-like properties.

Why Light Ejects Electrons from Metals

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key terms and concepts

Explore how light frees electrons from metals, why threshold frequency matters, and how Einstein’s photon idea explained the photoelectric effect.

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Why Light Ejects Electrons from Metals

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