Einstein's Explanation Of Photoelectric Effect - A Complete Guide

Einstein's Explanation Of Photoelectric Effect - A Complete Guide

Edited By Team Careers360 | Updated on Jul 02, 2025 05:02 PM IST

Albert Einstein, one of the greatest physicists, contributed his inevitable works and proposed many theories in different fields of physics like the special theory of relativity, concepts of the black hole, photoelectric effect, Einstein quantum theory, Einstein-Maxwell equation, many more topics and some other assumptions. His instinct on the theories never fails. In any field, the Albert Einstein equation plays a prominent role and the Einstein photoelectric equation can be considered as the best achievement of his work. In 1921, he received the Nobel Prize in Physics for his work Einstein theory of photoelectric effect.

This Story also Contains
  1. What is Einstein Theory of Photoelectric Effect?
  2. Einstein Explanation of Photoelectric effect:
  3. Photoelectric Effect Derivation
  4. Numerical Problems on Photoelectric Effect
Einstein's Explanation Of Photoelectric Effect - A Complete Guide
Einstein's Explanation Of Photoelectric Effect - A Complete Guide

In this article, we will discuss about the Einstein photoelectric equation, derive the equation for the photoelectric effect. We'll also cover important related terms, such as threshold energy, work function, and stopping potential, with related solved numerical examples.

What is Einstein Theory of Photoelectric Effect?

The Einstein theory of photoelectric effect defines that- "Electrons are ejected from the surface of the metal when a ray of light incident on it." These ejected electrons are termed as Photoelectrons. The amount of electrons ejected from the from the surface will depends on the frequency of light incident on it. Let us discuss the explanation of Photoelectric effect.

Einstein Explanation of Photoelectric effect:

Now, let us elaborate and explain the Einstein photoelectric equation as shown in the following diagram:

  • The wavy red lines represent light particles that is striking the metal surface. Energy of these photons is determined by Plank's equation:

$E=h \nu$. Where, $h$ is Planck's constant $\left(6.626 \times 10^{-34} \mathrm{Js}\right)$, $\nu$ (or $f$ ) is the frequency of the electromagnetic wave.

  • When the light particle (photon) hits the metal surface it transfers it energy to the electrons present on the metal surface. If this energy is greater than the work function $(\phi)$ of the metal, then electrons overcome the attractive forces holding it in the metal.
  • The electrons will absorb the energy and leave the metal surface, as shown by the arrow directed away from the surface. These arrow represents that the electrons are leaving with Kinetic energy.
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 Photoelectric effect

In Brief we can say that the electron will leave the metal surface if the energy provided by the photons will be greater than the work function of the metal.

If the energy provided by the photons are less than the work function, no electron is released from the metal surface.

The laws of Photoelectric Emission:

  • There is a minimum cutoff frequency required for emission of electrons. Below this frequency, no electron emission occurs
  • The total number of emitted electrons increases with the intensity of incident light
  • Kinetic energy of the electrons emitted is dependent on the frequency of light and independent of intensity of light
  • There are zero time lags between the incidence of the light beam and electron emission.

Photoelectric Effect Equation

Einstein photoelectric effect equation is given by:

$$
\mathrm{KE}_{\max }=h \nu-\phi
$$

where:

  • $\mathrm{KE}_{\max }$ is the maximum kinetic energy of the ejected electrons,
  • $h$ is Planck's constant ( $6.626 \times 10^{-34} \mathrm{~J}$ s),
  • $\nu$ (or $f$ ) is the frequency of the incident light,
  • $\phi$ is the work function of the metal, which is the minimum energy needed to release an electron from the metal's surface.

Also read -

Photoelectric Effect Derivation

Let us derive Einstein’s photoelectric equation mathematically and this can be written as

Energy of photon = Energy required to remove the electron from the surface of the metal (W) + Maximum Kinetic energy of the electron which is ejected from the surface of the metal.

The energy of the incident photon ( $E=h \nu$ ) is used in two parts:
- To remove the electron from the metal surface, which requires the work function $W$.
- To provide kinetic energy K. E. to the ejected electron.

Therefore, we can write:

$$
h \nu=W+\mathrm{K} . \mathrm{E}
$$


Rearranging, we get:

$$
\mathrm{K} . \mathrm{E} .=h \nu-W
$$

At the threshold frequency $\nu_0$, the photon's energy is just enough to overcome the work function without imparting any kinetic energy to the electron. Thus:

$$
W=h \nu_0
$$

3. Maximum Kinetic Energy:

Substituting $W=h \nu_0$ into the kinetic energy equation, we get:

$$
\mathrm{K} . \mathrm{E}_{\cdot \max }=h \nu-h \nu_0
$$

or

$$
\mathrm{K} . \mathrm{E}_{\cdot \max }=h\left(\nu-\nu_0\right)
$$


For an electron with mass $m$ and maximum velocity $v_{\text {max }}$, the kinetic energy can be expressed as:

$$
\mathrm{K} . \mathrm{E}_{\max }=\frac{1}{2} m v_{\max }^2=h\left(\nu-\nu_0\right) ----(1) $$

The stopping potential $V_0$ is the potential required to bring the ejected electrons to a halt.
Thus:

$$
e V_0=\frac{1}{2} m v_{\max }^2
$$

Substituting from equation (1), we get:

$$
e V_0=h\left(\nu-\nu_0\right) ----(2) $$

This equation represents the relationship between the stopping potential $V_0$, the frequency of the incident light $\nu$, and the threshold frequency $\nu_0$ :

$$
e V_0=h\left(\nu-\nu_0\right)
$$
This final expression shows the Einstein photoelectric effect equation, showing how the maximum kinetic energy (or the stopping potential) depends on the frequency of the incident light and the material’s threshold frequency.

Scientific Terms in Photoelectric Effect

Threshold Frequency:

The minimum frequency of the given incident light beam which is required for the emission of electrons from the surface of the metal is known as Threshold frequency.

Work Function:

The minimum energy required in the removal of the electron from the surface of the metal is known as Work Function. It is represented by $\phi$

Stopping potential:

The required potential to stop the emission of the electron from the surface of the metal is known as stopping potential. It is represented by $V_0$

Numerical Problems on Photoelectric Effect


1. The threshold frequency for a certain metal is $5 \times 10^{14} \mathrm{~Hz}$. Calculate the work function of the metal. (Use $h=6.626 \times 10^{-34} \mathrm{~J}$ ).

Solution:
1. Use the formula for the work function $(\phi)$ in terms of the threshold frequency $\left(\nu_0\right)$ :

$$
\phi=h \nu_0
$$

where $h=6.626 \times 10^{-34} \mathrm{~J}_{\mathrm{s}}$ and $\nu_0=5 \times 10^{14} \mathrm{~Hz}$.
2. Substitute the values:

$$
\phi=\left(6.626 \times 10^{-34} \mathrm{~J} s\right) \times\left(5 \times 10^{\mathbf{1 4}} \mathrm{Hz}\right)=3.313 \times 10^{-19} \mathrm{~J}
$$

3. Convert this to electron volts ( eV ) by dividing by the charge of an electron, $e=1.6 \times 10^{-19} \mathrm{C}$ :

$$
\phi=\frac{3.313 \times 10^{-19} \mathrm{~J}}{1.6 \times 10^{-19} \mathrm{~J} / \mathrm{eV}}=2.07 \mathrm{eV}
$$
Answer: The work function of the metal is 2.07 eV .

2- Light of wavelength 400 nm is incident on a metal surface with a work function of 2.2 eV . Will electrons be ejected from the metal surface? If yes, calculate the maximum kinetic energy of the emitted electrons. (Use $h=6.626 \times 10^{-34} \mathrm{Js}^{-}, c=3 \times 10^8 \mathrm{~m} / \mathrm{s}$ ).

Solution:
1. First, calculate the energy of the incident photon using $E=\frac{h c}{\lambda}$ :

$$
\begin{aligned}
E & =\frac{\left(6.626 \times 10^{-34} \mathrm{~J}\right) \times\left(3 \times 10^8 \mathrm{~m} / \mathrm{s}\right)}{400 \times 10^{-9} \mathrm{~m}} \\
E & =\frac{1.9878 \times 10^{-25} \mathrm{~J}}{4 \times 10^{-7} \mathrm{~m}}=4.97 \times 10^{-19} \mathrm{~J}
\end{aligned}
$$

2. Convert $E$ to electron volts:

$$
E=\frac{4.97 \times 10^{-19} \mathrm{~J}}{1.6 \times 10^{-19} \mathrm{~J} / \mathrm{eV}}=3.1 \mathrm{eV}
$$

3. Since the photon's energy ( 3.1 eV ) is greater than the work function ( 2.2 eV ), electrons will be ejected from the metal surface.
4. Calculate the maximum kinetic energy of the ejected electrons using the photoelectric equation:

$$
\mathrm{KE}_{\max }=E-\phi=3.1 \mathrm{eV}-2.2 \mathrm{eV}=0.9 \mathrm{eV}
$$
Answer: Yes, electrons will be ejected, and the maximum kinetic energy of the emitted electrons is 0.9 eV .

Frequently Asked Questions (FAQs)

1. State the laws of photoelectric emission.
  • There is a minimum cutoff frequency required for emission of electrons. Below this frequency, no electron emission occurs

  • The total number of emitted electrons increases with the intensity of incident light

  • Kinetic energy of the electrons emitted is dependent on the frequency of light and independent of intensity of light

  • There are zero time lags between the incidence of the light beam and electron emission.

2. State photoelectric effect or define photoelectric emission.

The Photoelectric effect can be explained by assuming that light is incident on the surface of the metal, and then the electron in the surface of the metal gets ejected. This phenomenon of photoelectric effect was explained by the Albert Einstein Photoelectric equation.

3. Who explained the photoelectric effect initially?

The photoelectric effect was initially explained by Heinrich Hertz in the year of 1887 and later it was preceded by Lenard in the year of 1902.

4. What is the Einstein photoelectric equation? (class 12)

The Einstein photoelectric equation can be written as follows


o= ½ (mvmax2) = h (ν - νo)

5. How to calculate threshold frequency?

The threshold frequency formula can be written as W= hνo, where h is Planck’s constant and h =  6.626  x 10-34 J Hz-1

6. How did Einstein's explanation of the photoelectric effect differ from classical physics?
Einstein proposed that light consists of discrete packets of energy called photons, contradicting the classical wave theory of light. This quantum approach explained why the energy of emitted electrons depends on the frequency of light, not its intensity.
7. What is the photoelectric effect?
The photoelectric effect is a phenomenon where electrons are emitted from a material when light shines on it. This effect demonstrates the particle nature of light, as it cannot be explained by classical wave theory.
8. Why doesn't increasing the intensity of light increase the kinetic energy of emitted electrons?
Increasing light intensity only increases the number of photons, not their individual energy. The kinetic energy of emitted electrons depends on the energy of individual photons, which is determined by the light's frequency, not its intensity.
9. What is a photon?
A photon is a discrete packet or quantum of electromagnetic energy. It is the elementary particle of light and all forms of electromagnetic radiation, exhibiting both wave and particle properties.
10. What is the significance of the threshold frequency in the photoelectric effect?
The threshold frequency is the minimum frequency of light required to cause photoelectric emission. Below this frequency, no electrons are emitted regardless of light intensity, as the photon energy is insufficient to overcome the work function.
11. How does Einstein's photoelectric equation relate photon energy to electron kinetic energy?
Einstein's photoelectric equation states that the maximum kinetic energy of emitted electrons is equal to the photon energy minus the work function: KEmax = hf - φ, where h is Planck's constant, f is the light frequency, and φ is the work function.
12. What is the stopping potential in a photoelectric experiment?
The stopping potential is the minimum voltage required to prevent the most energetic photoelectrons from reaching the collector in a photoelectric cell. It's used to determine the maximum kinetic energy of the emitted electrons.
13. What role does Planck's constant play in the photoelectric effect?
Planck's constant (h) appears in the equation for photon energy (E = hf) and in Einstein's photoelectric equation. It quantifies the relationship between a photon's frequency and its energy, playing a crucial role in explaining the quantized nature of light.
14. What is the work function in the context of the photoelectric effect?
The work function is the minimum energy required to remove an electron from the surface of a material. It represents the energy barrier that must be overcome for photoelectric emission to occur.
15. How does the material of the photocathode affect the photoelectric effect?
Different materials have different work functions, affecting the threshold frequency and the efficiency of electron emission. Materials with lower work functions are more sensitive to lower frequency light and generally emit electrons more easily.
16. How does the energy of a photon relate to the frequency of light?
The energy of a photon is directly proportional to the frequency of light. This relationship is expressed by Einstein's equation: E = hf, where E is the photon energy, h is Planck's constant, and f is the frequency of light.
17. What is the significance of the cutoff wavelength in the photoelectric effect?
The cutoff wavelength is the longest wavelength (or lowest frequency) of light that can cause photoelectric emission for a given material. It corresponds to the threshold frequency and is determined by the work function of the material.
18. How does temperature affect the photoelectric effect?
Temperature has minimal direct effect on the photoelectric effect itself. However, at higher temperatures, some electrons may have enough thermal energy to overcome the work function with less photon energy, slightly lowering the effective threshold frequency.
19. Why doesn't the photoelectric effect occur with all materials and all frequencies of light?
The photoelectric effect only occurs when the photon energy (hf) exceeds the work function (φ) of the material. Different materials have different work functions, and lower frequency light may not have enough energy to cause electron emission.
20. How does the photoelectric effect relate to the concept of quantization in quantum mechanics?
The photoelectric effect demonstrates energy quantization in light (photons) and in matter (discrete electron energy levels). This quantization is a fundamental principle of quantum mechanics, contrasting with the continuous energy concept in classical physics.
21. Why was Einstein awarded the Nobel Prize for his explanation of the photoelectric effect?
Einstein's explanation resolved the contradictions between experimental observations and classical physics predictions. By introducing the concept of light quanta (photons), he provided a revolutionary understanding of light's nature, laying groundwork for quantum mechanics.
22. How does the photoelectric effect demonstrate the particle nature of light?
The photoelectric effect shows that light energy is transferred in discrete amounts (photons) rather than continuously. This quantized energy transfer is a key characteristic of particles, supporting the particle nature of light.
23. How does the photoelectric effect relate to the wave-particle duality of light?
The photoelectric effect provides strong evidence for the particle nature of light, while other phenomena (like interference) demonstrate its wave nature. This dual behavior is encapsulated in the concept of wave-particle duality.
24. What is the significance of the instantaneous nature of the photoelectric effect?
The instantaneous emission of electrons upon light exposure supports the particle model of light. If light were purely a wave, there would be a time delay as the electrons absorbed enough energy to be emitted.
25. How does the photoelectric effect differ from the Compton effect?
While both demonstrate the particle nature of light, the photoelectric effect involves the complete absorption of a photon and emission of an electron, whereas the Compton effect involves the scattering of a photon by an electron, with the photon losing some energy but continuing to exist.
26. How does the photoelectric effect contribute to our understanding of quantum entanglement?
While the photoelectric effect itself doesn't directly involve entanglement, it laid the groundwork for understanding quantum phenomena. Entangled photon pairs, used in many quantum experiments, are detected using photoelectric-based single-photon detectors, linking these concepts in practical applications.
27. What is meant by the term "one-to-one correspondence" in the context of the photoelectric effect?
One-to-one correspondence refers to the fact that each photon interacts with only one electron in the photoelectric effect. This explains why increasing light intensity (more photons) increases the number of emitted electrons but not their individual energies.
28. How does the photoelectric effect demonstrate that light's energy depends on frequency, not amplitude?
In the photoelectric effect, the energy of emitted electrons depends on the frequency of incident light, not its intensity (which is related to amplitude). This observation contradicts classical wave theory and supports the quantum nature of light.
29. What is the relationship between the photoelectric effect and the development of photovoltaic cells?
The photoelectric effect is the underlying principle for photovoltaic cells. While traditional photoelectric devices emit electrons into vacuum, photovoltaic cells use the effect to generate electric current within a material, forming the basis of solar energy conversion.
30. How does the photoelectric effect relate to the concept of wave function collapse in quantum mechanics?
The photoelectric effect demonstrates the collapse of the light's wave function upon interaction with matter. The absorption of a photon by an electron represents a measurement event, causing the photon's wave-like probability distribution to collapse into a particle-like interaction.
31. What is the difference between external and internal photoelectric effects?
The external photoelectric effect involves electrons being emitted from a material's surface into vacuum or another medium. The internal photoelectric effect occurs when electrons are excited to higher energy levels within the material but not emitted, as in semiconductors.
32. How does the photoelectric effect contribute to our understanding of the nature of reality?
The photoelectric effect challenges our classical intuitions about the nature of light and matter. It demonstrates that at the quantum level, phenomena can exhibit both wave-like and particle-like properties, leading to a more nuanced understanding of physical reality.
33. What is the role of the photoelectric effect in modern technologies?
The photoelectric effect is utilized in various technologies, including solar cells, photomultiplier tubes, night vision devices, and photoelectric sensors. It forms the basis for converting light energy into electrical signals or power in many applications.
34. How does the concept of photon momentum relate to the photoelectric effect?
While the photoelectric effect primarily demonstrates the energy of photons, it also indirectly supports the concept of photon momentum. The conservation of momentum in photon-electron interactions is more directly observed in effects like the Compton scattering.
35. What is the significance of the linear relationship between stopping potential and light frequency in photoelectric experiments?
The linear relationship between stopping potential and light frequency provides direct experimental support for Einstein's photoelectric equation. The slope of this line gives Planck's constant, while the y-intercept relates to the work function of the material.
36. How does the photoelectric effect illustrate the limitations of the classical wave theory of light?
The photoelectric effect reveals several phenomena that classical wave theory cannot explain: the instantaneous nature of electron emission, the frequency dependence of electron energy, and the existence of a threshold frequency. These observations necessitated the development of quantum theory.
37. What is the connection between the photoelectric effect and the uncertainty principle?
The photoelectric effect demonstrates the particle-like behavior of light, which is complementary to its wave-like behavior. This duality is fundamental to the uncertainty principle, which states that certain pairs of physical properties (like position and momentum) cannot be simultaneously known with arbitrary precision.
38. How does the concept of virtual photons in quantum field theory relate to the photoelectric effect?
While the photoelectric effect involves real photons, the concept of virtual photons in quantum field theory extends our understanding of light-matter interactions. Virtual photons mediate electromagnetic forces between charged particles, providing a deeper explanation for phenomena like the photoelectric effect at the quantum field level.
39. What is the role of band theory in explaining the photoelectric effect in semiconductors?
Band theory helps explain the internal photoelectric effect in semiconductors. When a photon is absorbed, it can excite an electron from the valence band to the conduction band. This process is crucial for understanding photoconductivity and the operation of devices like photodiodes and solar cells.
40. How does the photoelectric effect relate to the concept of quantized atomic energy levels?
The photoelectric effect indirectly supports the concept of quantized atomic energy levels. The discrete nature of electron emission energies aligns with the idea that electrons occupy specific energy states in atoms, transitioning between these states through the absorption or emission of photons.
41. What is the significance of the photoelectric effect in the development of quantum electrodynamics (QED)?
The photoelectric effect was one of the key phenomena that led to the development of quantum mechanics, which in turn evolved into quantum electrodynamics. QED provides a complete quantum description of light-matter interactions, encompassing and extending the insights gained from the photoelectric effect.
42. How does the photoelectric effect demonstrate the failure of the equipartition theorem from classical statistical mechanics?
The equipartition theorem predicts that the energy of a system should be equally distributed among all degrees of freedom, which would allow electrons to gradually accumulate energy from light waves. The photoelectric effect shows this doesn't happen, supporting the quantized nature of energy transfer.
43. What is the relationship between the photoelectric effect and the concept of wave function in quantum mechanics?
The wave function in quantum mechanics describes the quantum state of a particle, including its probabilistic nature. In the photoelectric effect, the wave function of an electron collapses upon absorption of a photon, transitioning from a bound state to a free electron state.
44. How does the photoelectric effect relate to the concept of quantum tunneling?
While the photoelectric effect itself doesn't involve tunneling, both phenomena illustrate quantum behavior. Quantum tunneling allows particles to overcome potential barriers they classically couldn't, similar to how the photoelectric effect allows electron emission that classical physics couldn't explain.
45. What is the significance of the photoelectric effect in understanding the nature of chemical bonds?
The photoelectric effect contributed to the understanding that electrons in atoms occupy discrete energy levels. This concept is fundamental to explaining chemical bonding, as it relates to how electrons are shared or transferred between atoms to form molecules.
46. How does the photoelectric effect contribute to our understanding of the wave-particle duality of matter?
While the photoelectric effect primarily demonstrates the particle nature of light, it indirectly supports the wave-particle duality of matter. The quantized nature of electron emission suggests that matter, like light, can exhibit both wave-like and particle-like properties under different circumstances.
47. What is the role of the photoelectric effect in modern quantum optics experiments?
In quantum optics, the photoelectric effect is utilized in single-photon detectors, crucial for experiments involving quantum entanglement, quantum cryptography, and other quantum information applications. These detectors rely on the emission of electrons by individual photons.
48. How does the photoelectric effect relate to the concept of quantum efficiency in photonic devices?
Quantum efficiency in photonic devices refers to the ratio of emitted electrons (or electron-hole pairs in semiconductors) to incident photons. This concept, rooted in the photoelectric effect, is crucial for evaluating and improving the performance of devices like photodetectors and solar cells.
49. What is the connection between the photoelectric effect and the development of the laser?
While the photoelectric effect involves absorption of photons, the development of lasers relies on the inverse process: stimulated emission. Both phenomena demonstrate the quantized nature of light-matter interactions, fundamental to understanding and manipulating light at the quantum level.
50. How does the photoelectric effect contribute to our understanding of blackbody radiation?
The photoelectric effect and blackbody radiation were both pivotal in the development of quantum theory. While blackbody radiation demonstrated the quantization of energy in oscillators, the photoelectric effect showed the quantization of light itself, both challenging classical physics and leading to quantum mechanics.
51. What is the significance of the photoelectric effect in the field of spectroscopy?
The photoelectric effect underlies various spectroscopic techniques, particularly photoelectron spectroscopy. This method uses the effect to study the electronic structure of atoms and molecules by measuring the kinetic energy of emitted electrons, providing insights into chemical bonding and material properties.
52. How does the photoelectric effect relate to the concept of work in thermodynamics?
The work function in the photoelectric effect is analogous to the concept of work in thermodynamics. It represents the minimum energy required to move an electron from its bound state to a free state, similar to how work in thermodynamics involves energy transfer between systems.
53. What is the role of the photoelectric effect in understanding the ionization of atoms and molecules?
The photoelectric effect provides a model for understanding ionization processes. When the energy of an incident photon exceeds the ionization energy of an atom or molecule, it can cause the ejection of an electron, similar to the photoelectric emission from a metal surface.
54. What is the significance of the photoelectric effect in the development of quantum computing?
The photoelectric effect contributes to quantum computing through its role in photon detection. Quantum computers that use photons as qubits rely on single-photon detectors based on the photoelectric effect. This connection highlights the ongoing importance of Einstein's discovery in cutting-edge quantum technologies.
55. How does the photoelectric effect relate to the concept of vacuum energy in quantum field theory?
The photoelectric effect demonstrated the quantum nature of light, a concept further developed in quantum field theory. Vacuum energy, the energy of empty space filled with virtual particles, is a consequence of quantum fields. Both concepts challenge classical notions of empty space and energy, illustrating the profound impact of quantum physics on our understanding of the universe.

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