Photoelectric Effect: Definition, Examples, and Applications

Q: What is Einstein's photoelectric equation?

Ep=W0+KE&there4;hν=hν0+1/2mv2

Photoelectric Effect

Edited By Shivani Poonia | Updated on Jul 02, 2025 05:49 PM IST

The photoelectric effect is studied under the context of Atomic structure and the behavior of electrons in atoms when electrons interact with electromagnetic radiations. A lot of experiments were conducted to understand the behavior of atoms. It was one of the experiments conducted in which a metal surface was exposed to light radiation, and it was observed that electrons were emitted from the metal surface. This phenomenon is termed as Photoelectric effect. Quantum mechanics was one of the main experimental evidence of the photoelectric effect. It has shown that light may act in some cases as a wave, and alternatively as a particle (photon), thereby providing evidence for energy quantization.

This Story also Contains

Spectroscopy:
Chemical Reactions:
Photoelectric Effect: Photon Magic
Some Solved Examples
Conclusion

Photoelectric Effect

In this article, we will cover the concept of the Photoelectric effect. This concept falls under the category of Atomic structure, which is a crucial chapter in Class 11 chemistry. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more.

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Spectroscopy:

A branch of chemistry dealing with the interaction between matter and radiation is spectroscopy. As such, spectroscopy is relevant in the photoelectric effect, explaining how photons having specific energies are absorbed by atoms and molecules making transitions from one energy level to another.

In particular, the photoelectric effect is used in methods such as Photoelectron Spectroscopy (PES) and X-ray Photoelectron Spectroscopy (XPS) to determine how electron energies are distributed over samples. These methods provide information about the electronic structure and chemical composition of substances.

Chemical Reactions:

Light can initiate photochemical reactions that serve either as reactants or catalysts in some chemical processes. Such reactions involve the emission of light from a molecule after it has absorbed a photon, resulting in rearrangement in its electronic state.

Photoelectric Effect: Photon Magic

Whenever a metal surface is exposed to light radiation of suitable energy or frequency, it was observed that some electrons get ejected from the metal surface. This phenomenon is called as Photoelectric effect and the ejected electrons are called photoelectrons.

There were certain observations in the photoelectric effect experiment.

(1) There was a requirement of a minimum energy for each metal for the photoelectric effect to occur. This minimum energy is known as work function (W₀) and it can be closely associated with the ionization energy of the metal.

Corresponding to the work function, there is a minimum frequency of light required for the photoelectric effect. This minimum frequency is called the Threshold frequency.
Corresponding to the work function, there is a maximum wavelength of incident light above which the Photoelectric effect cannot occur. This maximum wavelength is called the Threshold wavelength.

Mathematically, the work function, threshold frequency, and threshold wavelength can be associated as

W₀=hν₀=hc/λ₀

Note: hc is approximately equal to 2 $\times$ 10^-25J-m or 12400 eV-nm. (eV is the energy in electron volts)

(2) The number of electrons ejected is proportional to the intensity (brightness) of light striking the metal but does not depend upon the frequency of light.

(3) There was almost no time lag between the striking of light and the ejection of photoelectrons

(4) The kinetic energy of the ejected electrons (photoelectrons) depends upon the frequency of the light used.

Einstein's photoelectric equation

From the conservation of energy

Ep=W₀+KE

∴hν=hν₀+1/2mv²

where

m is the mass of the electron

v is the velocity associated with the ejected electron.

h is Planck’s constant.

v is the frequency of the photon,

v₀ is the threshold frequency of metal.

(5) The Kinetic energy of an ejected photoelectron is also sometimes associated with Stopping Potential. It is defined as the minimum opposing potential applied due to which the kinetic energy of the electron becomes zero.

1/2mv²=eVs

where,

V_s = Stopping potential

e = Charge on electron

Some Solved Examples

Example 1:

Light of wavelength λ shines on a metal and emits Y electrons per second of average energy Z. What will happen to Y and Z if the intensity of light is doubled?

1)Y will be doubled and Z will be halved

2)Y will remain the same and Z will be doubled.

3)Both Y and Z will be doubled.

4) (correct)Y will increase but Z will remain the same.

Solution

When intensity is doubled, it means that the brightness of the light is doubled but the energy of the incident beam is kept the same.

So on increasing the number of incident photons, the number of electrons emitted per second will increase but the average energy of photoelectrons emitted remains the same.

Hence, the answer is the option (4).

Example 2:

Ejection of the photoelectron from metal in the photoelectric effect experiment can be stopped by applying 0.5 V when the radiation of 250 nm is used. The work function (eV) of the metal is :

1)4

2) (correct)4.5

3)5

4)5.5

Solution

λ=250 nm=2500

Ep=hc/λ=12400/2500=4.96eV

KE=e×Vs=e×0.5 V=0.5eV

Ep=W₀+KE

4.96=W₀+0.5

W₀=4.46eV≈4.5eV

Hence, the answer is an option (2).

Example 3:

A metal surface is exposed to solar radiation

1) (correct)The emitted electrons have energy less than a maximum value of energy depending upon the frequency of the incident radiation.

2)The emitted electrons have energy less than a maximum value of energy depending upon the intensity of the incident radiation.

3)The emitted electrons have zero energy

4)The emitted electrons have energy equal to the energy of photons of the incident light.

Solution

As we have learned in the photoelectric effect,

Energy of Photon = Work Function +KEmax

Here, the expression KEmax represents the maximum possible Kinetic energy of the emitted photoelectron under ideal conditions i.e. assuming no losses.

This implies that, under real (non-ideal) conditions, the Kinetic energy of the electron will be lesser than the value of KEmax.

It is also to be noted that the intensity affects the number of Photoelectrons emitted but does not affect their energy.

Hence, the answer is the option (1).

Example 4 :

The metal mainly used in devising photoelectric cells is :

1)Na
2)Li

3)Rb

4) (correct)Cs

Solution

Cs are used in photoelectric cells as it has the least ionization energy.

Hence, the answer is the option(4).

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NCERT Chemistry Notes:

Conclusion

To sum up, the photoelectric effect is a foundation in both physics and chemistry, revealing the dual nature of light and providing profound explanations about electrons’ behaviour in atoms and molecules. Its finding led to the development of quantum mechanics, leading to significant advances in spectroscopy, material science, and energy conversion technologies. By explaining how photons interact with matter, the photoelectric effect still contributes to our knowledge of the tiny world and stimulates scientific progress in many fields. We also learned, In particular, the photoelectric effect is used in methods such as Photoelectron Spectroscopy (PES) and X-ray Photoelectron Spectroscopy (XPS) to determine how electron energies are distributed over samples. These methods provide information about the electronic structure and chemical composition of substances.

Frequently Asked Questions (FAQs)

1. What is stopping potential?

During the photoelectric effect, the minimum amount of potential energy required to stop the kinetic energy of the elections becomes zero.

2. In context with the photoelectric effect, how is the stopping potential related to the mass of the electrons?

About the photoelectric effect, the mass of the electron is directly proportional to the stopping potential.

3. What is Einstein's photoelectric equation?

Ep=W0+KE∴hν=hν0+1/2mv2

4. At the wavelength above which photoelectric effect does not occur?

The maximum wavelength above which the photoelectric effect does not occur is the Threshold wavelength.

5. Name any two methods where the application of the photoelectric effect can be utilized.

Photoelectron Spectroscopy (PES) and X-ray Photoelectron Spectroscopy (XPS) to determine how electron energies are distributed over samples. These methods provide information about the electronic structure and chemical composition of substances.

6. What is a photon in the context of the photoelectric effect?

A photon is a discrete packet or particle of electromagnetic energy. In the photoelectric effect, each photon interacts with a single electron, transferring its energy. The energy of a photon is given by E = hf, where h is Planck's constant and f is the frequency of the light.

7. How does the photoelectric effect relate to the quantization of energy?

The photoelectric effect demonstrates energy quantization because it shows that light energy is transferred in discrete amounts (photons) rather than continuously. Each photon has a specific energy determined by its frequency, and this energy is either completely absorbed by an electron or not at all, leading to the observed threshold frequency.

8. What role does the photoelectric effect play in solar cells?

Solar cells operate based on the photoelectric effect. When sunlight strikes the semiconductor material in a solar cell, it causes electrons to be excited to a higher energy state. These electrons can then flow as an electric current, converting light energy into electrical energy. The efficiency of solar cells depends on matching the semiconductor's properties to the solar spectrum.

9. Why can't the photoelectric effect be explained by treating light solely as a wave?

If light were solely a wave, its energy would be spread out and increase with intensity. This would mean that: 1) Any frequency of light should eventually cause electron emission if intense enough. 2) There would be a time delay between light striking the surface and electron emission. 3) Electron energy would increase with light intensity. None of these are observed, contradicting the wave-only model.

10. How does the photoelectric effect demonstrate the failure of classical physics?

The photoelectric effect reveals several failures of classical physics: 1) It shows that light energy is quantized, not continuous. 2) It demonstrates that light can behave as particles, not just waves. 3) It shows that electron energies in atoms are quantized. These observations led to the development of quantum mechanics, a fundamental shift from classical physics.

11. What is the photoelectric effect?

The photoelectric effect is a phenomenon where electrons are emitted from a material when light shines on it. It occurs when photons (particles of light) transfer their energy to electrons in the material, causing them to be ejected if the energy is sufficient.

12. Who discovered the photoelectric effect?

Heinrich Hertz first observed the photoelectric effect in 1887 during his experiments with radio waves. However, Albert Einstein later explained the phenomenon in 1905, which earned him the Nobel Prize in Physics in 1921.

13. How does the photoelectric effect contradict classical wave theory of light?

The photoelectric effect contradicts classical wave theory in several ways: 1) It occurs instantly, regardless of light intensity. 2) Electron emission depends on light frequency, not intensity. 3) There's a threshold frequency below which no electrons are emitted, regardless of intensity. These observations can only be explained by treating light as particles (photons) rather than waves.

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 a material's surface. It's specific to each material and represents the energy needed to overcome the electron's binding to the atom. In the photoelectric effect, incident photons must have energy greater than the work function to eject electrons.

15. How does Einstein's equation for the photoelectric effect relate energy and frequency?

Einstein's photoelectric equation is E = hf - φ, where E is the kinetic energy of the ejected electron, h is Planck's constant, f is the frequency of the incident light, and φ is the work function of the material. This equation shows that the electron's energy depends directly on the light's frequency, not its intensity.

16. What is the relationship between photon energy and light color?

Photon energy is directly related to light frequency, which corresponds to color for visible light. Higher frequency (shorter wavelength) light like blue or violet has higher energy photons than lower frequency (longer wavelength) light like red or orange. This is why ultraviolet light, which has even higher frequency than visible light, can cause sunburn.

17. How does the photoelectric effect explain line spectra of elements?

While the photoelectric effect doesn't directly explain line spectra, both phenomena demonstrate the quantized nature of electron energy levels in atoms. In the photoelectric effect, electrons are ejected when photons have sufficient energy. In line spectra, electrons transition between specific energy levels, emitting or absorbing photons of specific frequencies.

18. What is the difference between the photoelectric effect and photoconductivity?

While both involve light interacting with electrons in materials, they differ in mechanism and outcome. The photoelectric effect involves electrons being completely ejected from a material by high-energy photons. Photoconductivity involves electrons being excited to a conductive state within the material by lower-energy photons, increasing the material's electrical conductivity without electron emission.

19. How does the photoelectric effect relate to Compton scattering?

Both the photoelectric effect and Compton scattering provide evidence for the particle nature of light. While the photoelectric effect involves photons transferring all their energy to bound electrons, Compton scattering involves photons colliding with free or loosely bound electrons, transferring only part of their energy and changing direction. Both phenomena support the concept of photons as particles.

20. What is the significance of the photoelectric effect in the development of quantum mechanics?

The photoelectric effect was crucial in the development of quantum mechanics because: 1) It provided evidence for the quantization of light energy. 2) It supported the particle nature of light, leading to the concept of wave-particle duality. 3) It demonstrated the quantized nature of electron energy levels in atoms. These insights were fundamental in formulating the principles of quantum mechanics.

21. What is the significance of the threshold frequency in the photoelectric effect?

The threshold frequency is the minimum frequency of light required to cause electron emission from a material. Below this frequency, no electrons are emitted regardless of light intensity. It's directly related to the work function: fₜ = φ/h, where fₜ is the threshold frequency, φ is the work function, and h is Planck's constant.

22. How does light intensity affect the photoelectric effect?

Light intensity affects the number of electrons emitted, not their energy. Higher intensity means more photons, which can eject more electrons if their energy exceeds the work function. However, the energy of individual electrons depends only on the light's frequency, not its intensity.

23. Why doesn't increasing light intensity allow electron emission below the threshold frequency?

Increasing light intensity only increases the number of photons, not their individual energy. If a photon's energy (determined by its frequency) is below the work function, it can't eject an electron regardless of how many such photons strike the material. This demonstrates light's particle-like behavior.

24. How does the photoelectric effect provide evidence for the particle nature of light?

The photoelectric effect demonstrates light's particle nature through: 1) The instantaneous nature of electron emission. 2) The dependence on frequency rather than intensity for electron ejection. 3) The existence of a threshold frequency. These observations can only be explained if light behaves as discrete particles (photons) with specific energies.

25. How does the photoelectric effect relate to the wave-particle duality of light?

The photoelectric effect is a prime example of light's wave-particle duality. While light exhibits wave-like properties in phenomena like diffraction and interference, the photoelectric effect can only be explained by treating light as particles (photons). This dual nature is a fundamental concept in quantum mechanics.

26. What is the stopping potential in a photoelectric experiment?

The stopping potential is the minimum voltage required to prevent the most energetic ejected electrons from reaching the collector in a photoelectric cell. It's used to determine the maximum kinetic energy of the emitted electrons, which is directly related to the frequency of the incident light.

27. How does changing the metal in a photoelectric experiment affect the results?

Changing the metal affects the work function, which in turn changes the threshold frequency and the kinetic energy of ejected electrons. Different metals will have different minimum frequencies of light required to eject electrons and will produce electrons with different maximum kinetic energies for the same incident light.

28. Why doesn't classical wave theory explain the instantaneous nature of the photoelectric effect?

Classical wave theory suggests that light energy is spread out over the wavefront. It would take time for an electron to accumulate enough energy to be ejected, leading to a time delay. However, the photoelectric effect occurs instantly, which can only be explained if light energy is concentrated in discrete packets (photons).

29. How does the photoelectric effect relate to Einstein's theory of relativity?

While the photoelectric effect doesn't directly relate to relativity, Einstein's explanation of it was crucial in the development of quantum theory. Both the photoelectric effect and relativity challenged classical physics and were part of the revolution in physics in the early 20th century. Einstein's work on the photoelectric effect also introduced the concept of light quanta, later called photons.

30. What is the significance of Planck's constant in the photoelectric effect?

Planck's constant (h) is crucial in the photoelectric effect as it relates the energy of a photon to its frequency: E = hf. It appears in Einstein's photoelectric equation and determines the relationship between the threshold frequency and work function. Its small value (approximately 6.63 × 10⁻³⁴ J·s) explains why quantum effects are not noticeable in everyday life.

31. How does the concept of wave-particle duality apply to electrons in the photoelectric effect?

While the photoelectric effect primarily demonstrates the particle nature of light, it also indirectly supports the wave-particle duality of electrons. The quantized energy levels of electrons in atoms (which determine the work function and threshold frequency) can only be fully explained by treating electrons as wave-like entities, as described by quantum mechanics.

32. What is the difference between the photoelectric effect and photoemission?

The terms are often used interchangeably, but strictly speaking, the photoelectric effect refers to the phenomenon of electron emission due to light absorption, while photoemission specifically refers to the emission of electrons from a material's surface due to light. Photoemission is essentially the observable manifestation of the photoelectric effect.

33. What is the role of electron affinity in the photoelectric effect?

Electron affinity, which is the energy released when an atom gains an electron, is related to but distinct from the work function in the photoelectric effect. Materials with lower electron affinity generally have lower work functions, making it easier for photons to eject electrons. However, the work function also depends on other factors like crystal structure and surface conditions.

34. How does temperature affect the photoelectric effect?

Temperature has a minor effect on the photoelectric effect. Increasing temperature slightly lowers the work function of a material, making it easier for electrons to be ejected. This is because thermal energy helps electrons overcome the potential barrier at the surface. However, the effect is usually small compared to the photon energies involved.

35. How does the photoelectric effect relate to the uncertainty principle?

The photoelectric effect doesn't directly demonstrate the uncertainty principle, but both are fundamental to quantum mechanics. The quantized nature of light and electron energy levels revealed by the photoelectric effect laid the groundwork for quantum theory, which includes the uncertainty principle. Both concepts challenge classical deterministic views of physics.

36. How does the work function relate to a material's electron configuration?

The work function is closely related to a material's electron configuration, particularly its outermost electrons. Materials with loosely bound outer electrons (like alkali metals) have lower work functions, while those with tightly bound outer electrons have higher work functions. This is why metals generally exhibit the photoelectric effect more readily than non-metals.

37. What is the role of surface conditions in the photoelectric effect?

Surface conditions significantly affect the photoelectric effect. Contamination, oxidation, or adsorbed gases can change the work function of a material. Clean, smooth surfaces generally have lower work functions and exhibit the photoelectric effect more efficiently. This is why photoelectric experiments often require ultra-high vacuum conditions.

38. How does the photoelectric effect relate to the concept of binding energy in atoms?

The work function in the photoelectric effect is closely related to the binding energy of the outermost electrons in atoms or solids. It represents the minimum energy needed to remove an electron from the material. In atoms, this is analogous to the ionization energy. The quantized nature of these energies, revealed by the photoelectric effect, reflects the discrete energy levels of electrons in atoms.

39. What is the significance of the photoelectric effect in modern technology?

The photoelectric effect has numerous applications in modern technology, including: 1) Photovoltaic cells for solar energy conversion. 2) Photoelectric sensors in automatic doors and light meters. 3) Photomultiplier tubes for detecting weak light signals. 4) Night vision devices. 5) Photoelectron spectroscopy for studying material properties. Its understanding is crucial in optoelectronics and quantum optics.

40. How does the photoelectric effect relate to the emission spectrum of a star?

While not directly related, both phenomena involve interactions between light and matter. The photoelectric effect demonstrates how atoms absorb light of specific frequencies to eject electrons. Similarly, a star's emission spectrum shows the specific frequencies of light emitted when electrons in excited atoms transition to lower energy levels. Both reflect the quantized nature of electron energy levels in atoms.

41. What is the difference between the photoelectric effect and thermionic emission?

Both processes involve electron emission from a material, but they differ in the energy source. The photoelectric effect is caused by incident light (photons) providing energy to eject electrons. Thermionic emission occurs when a material is heated, giving electrons enough thermal energy to overcome the work function. The photoelectric effect is instantaneous and depends on light frequency, while thermionic emission depends on temperature.

42. How does the photoelectric effect demonstrate the conservation of energy?

The photoelectric effect clearly demonstrates energy conservation. The energy of the incident photon (hf) is equal to the work function (φ) plus the kinetic energy of the ejected electron (KE): hf = φ + KE. This equation shows that the photon's energy is completely accounted for, being either used to overcome the work function or converted into the electron's kinetic energy.

43. What is the role of the photoelectric effect in photoemission spectroscopy?

Photoemission spectroscopy uses the photoelectric effect to study the electronic structure of materials. By measuring the kinetic energy of electrons ejected by photons of known energy, researchers can determine the binding energies of electrons in the material. This technique provides valuable information about a material's electronic, chemical, and atomic properties.

44. How does the photoelectric effect relate to the concept of quantized angular momentum in atoms?

While the photoelectric effect doesn't directly demonstrate quantized angular momentum, both concepts are fundamental to our understanding of atomic structure. The quantized energy levels revealed by the photoelectric effect are a consequence of electrons occupying specific orbitals in atoms, which have quantized angular momentum. This quantization is a key principle in quantum mechanics.

45. What is the significance of the photoelectric effect in understanding the dual nature of matter?

The photoelectric effect was crucial in establishing the dual nature of light, showing that it behaves both as a wave and as particles (photons). This concept of wave-particle duality was later extended to matter by de Broglie. The photoelectric effect thus played a key role in developing the fundamental principle that all matter and energy exhibit both wave and particle properties.

46. How does the photoelectric effect relate to the concept of quantum tunneling?

While the photoelectric effect and quantum tunneling are distinct phenomena, both demonstrate quantum mechanical behavior. The photoelectric effect shows how electrons can be ejected from a material by absorbing discrete quanta of energy (photons). Quantum tunneling, on the other hand, describes how particles can pass through potential barriers that they classically shouldn't be able to overcome. Both phenomena challenge classical physics and are explained by quantum mechanics.

47. What is the role of the photoelectric effect in understanding atomic orbitals?

The photoelectric effect provides indirect evidence for the existence of atomic orbitals. The quantized nature of electron ejection, particularly the existence of a threshold frequency, suggests that electrons occupy discrete energy levels in atoms. These energy levels correspond to different atomic orbitals. While the photoelectric effect doesn't directly map out orbital structures, it supports the quantum mechanical model of the atom.

48. How does the photoelectric effect relate to the concept of work in physics?

The work function in the photoelectric effect is closely related to the concept of work in physics. It represents the minimum amount of work (energy transfer) required to remove an electron from the material. The kinetic energy of the ejected electron is the excess energy provided by the photon beyond this work function. This demonstrates the principle of energy conservation in a quantum context.

49. What is the significance of the photoelectric effect in the development of the quantum theory of light?

The photoelectric effect was pivotal in developing the quantum theory of light. It provided concrete evidence that light energy is quantized, leading to the concept of photons. This challenged the classical wave theory of light and was a crucial step in the development of quantum mechanics. Einstein's explanation of the photoelectric effect was one of the founding contributions to quantum theory.