Photo-Electric-Effect

Photoelectric Effect with MCQs

Definition of Photo-Electric Effect

The emission of electrons from a metal surface area when the light of ideal frequency is incident on it is called the photoelectric effect The emitted electrons are called photoelectrons.

Historical Perspective of Photoelectric Effect

The photoelectric effect, found by Hertz in 1887, is the basis of some of the most effective techniques capable of examining the electronic structure of condensed-matter systems– the techniques collectively identified as “photoemission spectroscopy.” Even before the advancement of such methods, the photoelectric effect had a fundamental influence on the history of modern-day science.

Experiment by Heinrich Hertz

Hertz carried out an experiment in which a spark gap generator was used. The sparks produced between two small metal spheres in a transmitter induce sparks that jump between two different metal spheres in a receiver.

Hertz found that he could increase the level of sensitivity of his spark gap device by illuminating it with visible or ultraviolet light. Later research studies by J.J. Thomson showed that this increased sensitivity was the outcome of light pushing on electrons– a particle that he discovered in 1897.

The primary turning points after Hertz’s work were Thompson’s 1897 discovery of the electron, Einstein’s 1905 hypothesis on the photon, and Millikan’s 1918 reluctant experimental validation of Einstein’s hypothesis. Einstein derived the photon hypothesis with a purely thermodynamic argument and utilized it to predict the frequency threshold in the photoelectric effect, which was not experimentally confirmed till the late 1910s.

Working of Photoelectric Effect

The target substance works as the anode, which ends up being the emitter of photoelectrons when it is lit up by monochromatic radiation. We call this electrode the photoelectrode. Photoelectrons are collected at the cathode, which is kept at a lower potential with respect to the anode.

The potential difference between the electrodes can be increased or reduced, or its polarity can be reversed. The electrodes are enclosed in an evacuated glass tube so that photoelectrons do not lose their kinetic energy on collisions with air particles in the area in between electrodes.

Working-Photo-electric

When the target material is not exposed to radiation, no current is observed in this circuit due to the fact that the circuit is broken (because there is a gap in between the electrodes). But when the target material is connected to the negative terminal of a battery and exposed to radiation, a current is observed in this circuit; this presentation is called the photocurrent.

Suppose that we now reverse the potential difference between the electrodes so that the target substance now gets in touch with the positive terminal of a battery, and after that, we slowly increase the voltage. The photocurrent slowly dies out and eventually stops flowing totally at some value of this reversed voltage.

Maximum Energy of Photoelectrons

The maximum energy of electrons can be figured out by reversing the terminals of the battery in the circuit. In this case, the photoelectrons are repelled by anode A and the photoelectrical current decreases. If the potential is made more and more negative then photoelectric current approaches to absolutely zero. Even the electrons of maximum energy are not able to reach the collector plate.

So, the maximum energy of photoelectron is,

K.E (max) = Ve

½ mv2 = Ve

Results of Experiment
  1. The electrons are emitted with different energies.
  2. The maximum energy of photoelectrons depends upon the surface area of metal and the frequency of incident light.
  3. There is a minimum frequency below which no photon emission happens.
  4. Electrons are discharged spontaneously.
  5. The variety of released photoelectrons relies on the intensity of light.
Further Reading:  Centripetal Force [with MCQs]

Results-photo-electric

Stopping Potential

The negative potential of the anode at which the photoelectric current becomes absolutely zero is called the stopping potential.

Threshold Frequency

The minimum value of frequency of incident light, at which electrons are emitted from a metal surface area, is called threshold frequency.

It varies from metal to metal and denoted by Fo.

MCQs

  • What is the photoelectric effect?
    • A) Emission of photons from a metal surface
    • B) Emission of electrons from a metal surface when illuminated with light of suitable frequency
    • C) Absorption of light by a metal surface
    • D) Reflection of light by a metal surface
    • Answer: B
  • Who discovered the photoelectric effect?
    • A) Albert Einstein
    • B) Heinrich Hertz
    • C) J.J. Thomson
    • D) Robert Millikan
    • Answer: B
  • What is the target substance called in the photoelectric effect setup?
    • A) Cathode
    • B) Anode
    • C) Photoelectrode
    • D) Cathode Ray Tube
    • Answer: C
  • What happens to the sensitivity of a spark gap device when illuminated with visible or ultraviolet light?
    • A) Decreases
    • B) Unaffected
    • C) Increases
    • D) Becomes zero
    • Answer: C
  • What term is used for the potential at which the photoelectric current becomes zero?
    • A) Stopping potential
    • B) Threshold potential
    • C) Maximum potential
    • D) Excitation potential
    • Answer: A
  • Which law relates the maximum energy of photoelectrons to the stopping potential?
    • A) Ohm’s Law
    • B) Boyle’s Law
    • C) Einstein’s Law
    • D) None of the above
    • Answer: D
  • What is the maximum energy of photoelectrons when the stopping potential is Vᵒ?
    • A) ½ mv² = Vᵒ e
    • B) ½ mv² = 2Vᵒ e
    • C) ½ mv² = Vᵒ
    • D) mv² = Vᵒ e
    • Answer: C
  • What does the threshold frequency represent in the photoelectric effect?
    • A) Maximum frequency of incident light
    • B) Minimum frequency of incident light
    • C) Average frequency of incident light
    • D) Frequency of emitted electrons
    • Answer: B
  • What determines the maximum energy of photoelectrons in the photoelectric effect?
    • A) Intensity of light
    • B) Voltage applied between electrodes
    • C) Threshold frequency of incident light
    • D) Surface area of the metal and frequency of incident light
    • Answer: D
  • What does the experiment by Heinrich Hertz with a spark gap generator demonstrate?
    • A) Emission of electrons from a metal surface
    • B) Reflection of light from a metal surface
    • C) Generation of electricity from light
    • D) Increase in the sensitivity of spark gap device when illuminated
    • Answer: D
  • What is the name of the electrons emitted when a metal surface is illuminated with light of suitable frequency?
    • A) Cathode rays
    • B) Anode rays
    • C) Photoelectrons
    • D) Beta particles
    • Answer: C
  • What is the characteristic property of the threshold frequency?
    • A) It varies from metal to metal
    • B) It remains constant for all metals
    • C) It is independent of the intensity of light
    • D) It depends on the surface area of the metal
    • Answer: A
  • What happens to the photoelectric current when the potential difference between electrodes is reversed?
    • A) Increases
    • B) Decreases
    • C) Remains constant
    • D) Becomes zero
    • Answer: B
  • What do the results of the photoelectric effect experiment suggest about the emission of electrons?
    • A) Emission with same energy
    • B) Emission with varying energies
    • C) Emission with constant energy
    • D) Absence of electron emission
    • Answer: B
  • What phenomenon determines the maximum energy of photoelectrons in the photoelectric effect?
    • A) Stopping potential
    • B) Threshold frequency
    • C) Intensity of light
    • D) Voltage applied between electrodes
    • Answer: A
  • What term is used for the potential at which the photoelectric current becomes zero?
    • A) Stopping potential
    • B) Threshold potential
    • C) Maximum potential
    • D) Excitation potential
    • Answer: A
  • What does the threshold frequency represent in the photoelectric effect?
    • A) Maximum frequency of incident light
    • B) Minimum frequency of incident light
    • C) Average frequency of incident light
    • D) Frequency of emitted electrons
    • Answer: B
Further Reading:  What is Torque? Definition, Formula and more

 

Frequently Asked Questions (FAQs) about the Photoelectric Effect:

  1. What is the photoelectric effect?
    • The photoelectric effect refers to the emission of electrons from a metal surface when illuminated with light of suitable frequency. The emitted electrons are called photoelectrons.
  2. Who discovered the photoelectric effect?
    • The photoelectric effect was discovered by Heinrich Hertz in 1887.
  3. How did Heinrich Hertz conduct his experiment on the photoelectric effect?
    • Hertz conducted an experiment using a spark gap generator, observing sparks between metal spheres induced by light, which increased the device’s sensitivity when illuminated with visible or ultraviolet light.
  4. What is the significance of the photoelectric effect in modern science?
    • The photoelectric effect laid the foundation for techniques like photoemission spectroscopy, providing insight into the electronic structure of condensed-matter systems.
  5. How does the photoelectric effect work?
    • In the photoelectric effect, a target substance (anode) emits photoelectrons when illuminated with monochromatic radiation. These photoelectrons are collected at the cathode, generating a photocurrent.
  6. What determines the maximum energy of photoelectrons in the photoelectric effect?
    • The maximum energy of photoelectrons depends on the surface area of the metal and the frequency of incident light.
  7. What is stopping potential in the context of the photoelectric effect?
    • Stopping potential is the negative potential at which the photoelectric current becomes zero.
  8. What is threshold frequency, and why is it important?
    • Threshold frequency is the minimum frequency of incident light required to emit electrons from a metal surface. It varies among different metals and is crucial in understanding the behavior of the photoelectric effect.
  9. How is the maximum energy of photoelectrons determined experimentally?
    • The maximum energy of photoelectrons can be determined by reversing the terminals of the battery in the circuit, observing the potential at which the photoelectric current approaches zero.
  10. What are the main results observed in experiments on the photoelectric effect?
    • The main results include emission of electrons with different energies, dependence of maximum energy on surface area and incident light frequency, existence of a minimum frequency for photon emission, and variation in released photoelectrons with light intensity.
  11. Can the photoelectric effect occur spontaneously?
    • Yes, electrons can be spontaneously emitted from a metal surface when illuminated with light of sufficient frequency.
  12. How does the threshold frequency vary among different metals?
    • The threshold frequency varies from metal to metal, and it is denoted by Fo. It depends on the specific characteristics of each metal.
  13. What happens to the photocurrent as the potential difference between electrodes is reversed?
    • The photocurrent decreases and eventually stops flowing completely at a certain reversed voltage.
  14. What role did J.J. Thomson and Albert Einstein play in the understanding of the photoelectric effect?
    • J.J. Thomson’s discovery of electrons in 1897 and Albert Einstein’s photon hypothesis in 1905 significantly contributed to the understanding and validation of the photoelectric effect.
  15. Why is the study of the photoelectric effect important in modern physics?
    • The photoelectric effect plays a crucial role in understanding quantum mechanics, the behavior of matter at the atomic and subatomic levels, and various applications in technology, including solar cells and photodetectors.

 

Summary: Photoelectric Effect Tutorial

The photoelectric effect, a phenomenon discovered by Heinrich Hertz in 1887, has had a profound impact on modern science and technology. It refers to the emission of electrons from a metal surface when illuminated with light of suitable frequency. Here’s a brief overview of the key aspects covered in this tutorial:

  1. Definition and Historical Perspective:
    • The photoelectric effect involves the emission of electrons, known as photoelectrons, from a metal surface under the influence of light. It played a crucial role in the development of photoemission spectroscopy and had significant implications for the understanding of quantum mechanics.
  2. Experiment by Heinrich Hertz:
    • Hertz conducted experiments using a spark gap generator, observing increased sensitivity when the device was illuminated with visible or ultraviolet light. Later discoveries by J.J. Thomson and Albert Einstein further elucidated the nature of light and electrons.
  3. Working Principle:
    • In the photoelectric effect, a target substance (anode) emits photoelectrons when illuminated by monochromatic radiation. These electrons are collected at the cathode, generating a photocurrent. The potential difference between electrodes affects the flow of electrons.
  4. Maximum Energy of Photoelectrons:
    • The maximum energy of photoelectrons can be determined by reversing the terminals of the battery in the circuit. This energy depends on factors such as the surface area of the metal and the frequency of incident light.
  5. Results of Experiment:
    • Experiments on the photoelectric effect revealed that electrons are emitted with different energies, depending on the metal surface and incident light frequency. There exists a minimum frequency, called the threshold frequency, below which no photon emission occurs.
  6. Stopping Potential and Threshold Frequency:
    • The stopping potential is the negative potential at which the photoelectric current becomes zero, while the threshold frequency is the minimum frequency of incident light required to emit electrons from a metal surface.
Further Reading:  Diffraction Grating: Introduction, Equation and Purpose

Overall, the photoelectric effect continues to be a subject of study and has numerous applications in various fields, including solar energy conversion, photodetectors, and semiconductor devices. Understanding its principles is essential for advancing our knowledge of light-matter interactions and quantum phenomena.