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What is photoelectric effect in chemistry?
When was the photoelectric effect first observed?
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Why does photoelectric effect occur at low light intensities?
Sep 12, 2022 · Figure 6.3.1: An experimental setup to study the photoelectric effect. The anode and cathode are enclosed in an evacuated glass tube. The voltmeter measures the electric potential difference between the electrodes, and the ammeter measures the photocurrent. The incident radiation is monochromatic.
- Overview
- Key points
- Introduction: What is the photoelectric effect?
- Predictions based on light as a wave
- When intuition fails: photons to the rescue!
- Light frequency and the threshold frequency ν0
- Isn't there more math somewhere?
- Exploring the wave amplitude trends
- Example 1 : The photoelectric effect for copper
- Example 2 : Calculating the kinetic energy of a photoelectron
Explaining the experiments on the photoelectric effect. How these experiments led to the idea of light behaving as a particle of energy called a photon.
•Based on the wave model of light, physicists predicted that increasing light amplitude would increase the kinetic energy of emitted photoelectrons, while increasing the frequency would increase measured current.
•Contrary to the predictions, experiments showed that increasing the light frequency increased the kinetic energy of the photoelectrons, and increasing the light amplitude increased the current.
•Based on these findings, Einstein proposed that light behaved like a stream of particles called photons with an energy of E=hν .
•The work function, Φ , is the minimum amount of energy required to induce photoemission of electrons from a metal surface, and the value of Φ depends on the metal.
When light shines on a metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. This process is also often referred to as photoemission, and the electrons that are ejected from the metal are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons. The prefix, photo-, simply tells us that the electrons have been ejected from a metal surface by incident light.
[Who discovered the photoelectric effect?]
To explain the photoelectric effect, 19th-century physicists theorized that the oscillating electric field of the incoming light wave was heating the electrons and causing them to vibrate, eventually freeing them from the metal surface. This hypothesis was based on the assumption that light traveled purely as a wave through space. (See this article for more information about the basic properties of light.) Scientists also believed that the energy of the light wave was proportional to its brightness, which is related to the wave's amplitude. In order to test their hypotheses, they performed experiments to look at the effect of light amplitude and frequency on the rate of electron ejection, as well as the kinetic energy of the photoelectrons.
Based on the classical description of light as a wave, they made the following predictions:
•The kinetic energy of emitted photoelectrons should increase with the light amplitude.
•The rate of electron emission, which is proportional to the measured electric current, should increase as the light frequency is increased.
To help us understand why they made these predictions, we can compare a light wave to a water wave. Imagine some beach balls sitting on a dock that extends out into the ocean. The dock represents a metal surface, the beach balls represent electrons, and the ocean waves represent light waves.
If a single large wave were to shake the dock, we would expect the energy from the big wave would send the beach balls flying off the dock with much more kinetic energy compared to a single, small wave. This is also what physicists believed would happen if the light intensity was increased. Light amplitude was expected to be proportional to the light energy, so higher amplitude light was predicted to result in photoelectrons with more kinetic energy.
When experiments were performed to look at the effect of light amplitude and frequency, the following results were observed:
•The kinetic energy of photoelectrons increases with light frequency.
•Electric current remains constant as light frequency increases.
•Electric current increases with light amplitude.
•The kinetic energy of photoelectrons remains constant as light amplitude increases.
These results were completely at odds with the predictions based on the classical description of light as a wave! In order to explain what was happening, it turned out that an entirely new model of light was needed. That model was developed by Albert Einstein, who proposed that light sometimes behaved as particles of electromagnetic energy which we now call photons. The energy of a photon could be calculated using Planck's equation:
We can think of the incident light as a stream of photons with an energy determined by the light frequency. When a photon hits the metal surface, the photon's energy is absorbed by an electron in the metal. The graphic below illustrates the relationship between light frequency and the kinetic energy of ejected electrons.
The scientists observed that if the incident light had a frequency less than a minimum frequency ν0 , then no electrons were ejected regardless of the light amplitude. This minimum frequency is also called the threshold frequency, and the value of ν0 depends on the metal. For frequencies greater than ν0 , electrons would be ejected from the metal. Furthermore, the kinetic energy of the photoelectrons was proportional to the light frequency. The relationship between photoelectron kinetic energy and light frequency is shown in graph (a) below.
We can analyze the frequency relationship using the law of conservation of energy. The total energy of the incoming photon, Ephoton , must be equal to the kinetic energy of the ejected electron, KEelectron , plus the energy required to eject the electron from the metal. The energy required to free the electron from a particular metal is also called the metal's work function, which is represented by the symbol Φ (in units of J ):
Ephoton=KEelectron+Φ
Like the threshold frequency ν0 , the value of Φ also changes depending on the metal. We can now write the energy of the photon in terms of the light frequency using Planck's equation:
Ephoton=hν=KEelectron+Φ
Rearranging this equation in terms of the electron's kinetic energy, we get:
KEelectron=hν−Φ
In terms of photons, higher amplitude light means more photons hitting the metal surface. This results in more electrons ejected over a given time period. As long as the light frequency is greater than ν0 , increasing the light amplitude will cause the electron current to increase proportionally as shown in graph (a) below.
Since increasing the light amplitude has no effect on the energy of the incoming photon, the photoelectron kinetic energy remains constant as the light amplitude is increased (see graph (b) above).
The work function of copper metal is Φ=7.53×10−19 J . If we shine light with a frequency of 3.0×1016 Hz on copper metal, will the photoelectric effect be observed?
In order to eject electrons, we need the energy of the photons to be greater than the work function of copper. We can use Planck's equation to calculate the energy of the photon, Ephoton :
Ephoton=hν=(6.626×10−34 J⋅s)(3.0×1016 Hz) plug in values for h and ν=2.0×10−17 J
If we compare our calculated photon energy, Ephoton , to copper's work function, we see that the photon energy is greater than Φ :
2.0×10−17 J > 7.53×10−19 J
Ephoton Φ
What is the kinetic energy of the photoelectrons ejected from the copper by the light with a frequency of 3.0×1016 Hz ?
We can calculate the kinetic energy of the photoelectron using the equation that relates KEelectron to the energy of the photon, Ephoton , and the work function, Φ :
Ephoton=KEelectron+Φ
Since we want to know KEelectron , we can start by rearranging the equation so that we will be solving for the kinetic energy of the electron:
KEelectron=Ephoton−Φ
Now we can insert our known values for Ephoton and Φ from Example 1:
The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation ( light ). Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, solid state, and quantum chemistry to draw inferences about the properties of atoms, molecules and solids.
The notion of light quantization was first introduced by Planck. Its validity is based on solid experimental evidence, most notably the photoelectric effect. The basic physical process underlying this effect is the emission of electrons in metals exposed to light.
Describe a typical photoelectric-effect experiment. Determine the maximum kinetic energy of photoelectrons ejected by photons of one energy or wavelength, when given the maximum kinetic energy of photoelectrons for a different photon energy or wavelength.
This phenomenon is known as the photoelectric effect. Electrons that are emitted in this process are called photoelectrons. The experimental setup to study the photoelectric effect is shown schematically in Figure 6.8. The target material serves as the cathode, which becomes the emitter of photoelectrons when it is illuminated by monochromatic ...
The photoelectric effect was first observed in 1887 by Heinrich Hertz during experiments with a spark gap generator (the earliest device that could be called a radio). In these experiments, sparks generated between two small metal spheres in a transmitter induce sparks that jump between between two different metal spheres in a receiver.
