Education Journalist | Study Abroad Lead
Dual Nature of Radiation and Matter is one of the easiest and scoring topics of Physics. This chapter is one of the important topics in JEE physics which, basically is an analysis of various nature of a matter. A matter can possess or display wave nature (exhibiting the phenomenon of interference and diffraction) and particle nature (quanta/packets of light). Therefore, by the dual nature of radiation and matter, i.e., electromagnetic radiation exhibits both phenomena. In this article, we will be discussing the basic concepts covered in this chapter and at the end will solve a few important questions.
| Table of Content |
Key Terms: Matter, radiation, photoelectric effect, dual nature of radiation and matter, Electromagnetic radiation, phenomena, wave nature, Light
What is Electron Emission and how does it happen?
[Click Here for Sample Questions]
Metals contain both electrons and protons. Electrons are bound inside the metal due to the attractive forces but with the help of energy, electrons can release themselves from the metal. This minimum requirement of energy to pull the electron from the metal is called the work function of the metal.
Certain methods lead to the emission of electrons from the metal surface. These methods are discussed below:
- Photo-electric emission: Requires light energy
Photo-electric emission
- Field emission: Requires electric field to release the electrons from the metal surface.
Field emission
- Thermionic emission: Requires thermal energy i.e. heat.
Thermonic Emission
Also Read:
Historical background of Photoelectric Effect
[Click Here for Sample Questions]
The photoelectric effect is the scientific phenomenon in which electromagnetic radiation occurs on the metal surface. This leads to the freeing of electrons from the metal surface by overpowering the attractive forces of protons. Thus, the electrons are released.
Due to the photoelectric effect, light energy gets converted into electrical energy, and the current produced due to the photoelectric effect is called photoelectric current.
- Hallwach and Lenard
As per their observations, it has been found that the electrons require a minimum frequency of light energy to be released from the metal surface; otherwise, attractive forces keep pulling the electrons towards the metal plate and won’t allow the expulsion of electrons. This minimum frequency is later termed as Threshold Frequency.
- Hertz
Heinrich Hertz observed that when the light of sufficient energy is incident on the metal surface, then two kinds of situations can be observed:
- Some electrons absorb the energy and overcome the attractive forces, thus start moving freely in the metal plate.
- But there are a few electrons that can also escape the metal surface and are found in the surrounding space.
Various factors that determine photoelectric current
[Click Here for Sample Questions]
Certain factors determine the photoelectric effect and in turn, also determine the photoelectric current. Let's discuss each of the factors in-depth:
Light Intensity
- Since it is a well-known fact that the photoelectric current depends on the number of electrons escaping the metal surface in one second.
- This indicates that the photoelectric current is in direct relation to the intensity of the light.
- So, the graph between photoelectric current and light intensity will be a straight line.
Light Intensity
Potential
- Stopping potential is the minimum provided on the metal plate that stops the photoelectric current.
- The minimum negative potential is the retarding potential as it is retarding the photoelectric current.
- The Kinetic Energy of the photoelectrons doesn’t depend on the intensity of the incident light.
Potential
Effect of frequency on the stopping potential
- The frequency of incident light is directly proportional to the maximum kinetic energy of the electrons.
- This indicates that there will be a greater requirement of retarding potential to stop the electrons from emitting out of the metal plate.
- Two important graphs to represent the same are given below:
Effect of frequency on the stopping potential
Effect of frequency on the stopping potential
Terminologies related to the dual nature of matter and radiation
[Click Here for Sample Questions]
- Free electrons are those which can freely move inside the metal plate but can’t escape the metal surface.
- Electron emission takes place when sufficient energy is provided to the metal surface which leads valence electrons to escape the metal plate and move in the surrounding space.
- Work Function is the minimum requirement of energy that helps to eject the electrons from the surface of the metal. The electron will keep on moving inside the metal though it has left its valence shell.
- The threshold frequency is the minimum frequency of light that can emit electrons from the metal surface.
Thing to Remember
[Click Here for Sample Questions]
- A matter can possess or display wave nature (exhibiting the phenomenon of interference and diffraction) and particle nature (quanta/packets of light).
- Metals contain both electrons and protons. Electrons are bound inside the metal due to the attractive forces but with the help of energy, electrons can release themselves from the metal.
- Certain methods lead to the emission of electrons from the metal surface. These methods are Photo-electric Emission, Field Emission, Thermionic Emission.
- The photoelectric effect is the scientific phenomenon in which electromagnetic radiation occurs on the metal surface.
- Due to the photoelectric effect, light energy gets converted into electrical energy, and the current produced due to the photoelectric effect is called photoelectric current.
Also Read:
Sample Questions
Ques 1) Define the term “intensity” in a photon picture of electromagnetic radiation. (CBSE 2019)
Ans. Electromagnetic radiation is the form of energy that is present around us invisibly. For instance, X-rays, radio waves, microwaves and so on. The intensity of electromagnetic radiation refers to the number of photons that passes through a given space.
Ques 2) Two protons of equal kinetic energies enter an area of equal magnetic field. The first proton enters normal to the field direction whereas the second enters at 30 degree to the field direction. What are the trajectories followed by them? (CBSE 2018)
Ans. As we know that when a charged particles enters a uniform magnetic field, the force extended on it is,
F = q(v x B) = qvb sinθ
In case 1, when the particle enters perpendicular, θ= 90
Here the particles start moving in a circular path with radius as sinθ = 1 is the maximum value.
In case 2 where a particle enters at an angle 30 degree, the force acting on it has two components. Thus, due to the resultant of the two components, the particle will move along a helical path.
Ques 3) Write one reason to explain why wave theory of light does not support the photoelectric effect. (Comptt. Delhi 2014)
Ans. One of the reasons why wave theory of light does not support photoelectric effect is that the kinetic energy of photoelectrons does not depend on the intensity of the incident light. Moreover, according to the wave theory, after the light falls on a substance, the electrons are emitted after a certain period of time. However, in photoelectric effect, electron emissions are immediate without a time delay.
Ques 4) Write two properties of photons. For a monochromatic radiation incident on a photosensitive surface, why all photoelectrons do not come out with the same energy? Give reason for your answer. (Comptt. All India 2017)
Ans. The two properties of photon are as follows:-
The photons are electrically neutral
Photon has an energy that is equivalent to hv.
For a monochromatic radiation incident on a photosensitive surface, all photoelectrons do not come out with the equal energy as in addition to the work done to the free electrons from surface, different emitted photoelectron require different amount of work to be done on them in order to reach the surface.
Ques 5) Draw a graph showing variation of stopping potential with frequency of incident radiations for two photosensitive materials A and B having threshold frequency VA > VB.
(i) In which case the stopping potential will be more and why?
(ii)Does the slope of the graph depend on the nature of the material that is used? Explain. (All India 2016)
Ans.
(i) For material B, because for the same value of ‘v’, the stopping potential is more for material B.
Therefore, V0 is higher for the lower value of v0
(ii) No, the slope of the graph does not depend on the nature of the material used as slope is given by h/e which is constant.
Ques 6) (a) State three observed features of photoelectric effect that can not be explained by wave theory of light. Explain how Einstein’s photoelectric equation is used to describe these features.
(b) The figure shows a plot of stopping potential V0 with frequency v of incident radiation for two photosensitive materials M1 and M2.
Answer the following questions:
(i) why is the slope of both the lines the same?
(ii) for which material emitted electrons have greater kinetic energy for the same frequency of the incident radiation? (Comptt. All India 2017)
Ans. (a) The three observed feature are as follows,
The maximum kinetic energy of the electrons that are emitted should be directly proportional to the intensity of incident radiations but it is not observed experimentally. Also maximum kinetic energy of the emitted electrons should not depend upon incident frequency based on the wave theory, but it is not so.
According to the wave theory, the threshold frequency should not be present. Light of all frequencies should emit electrons where intensity of light is sufficient for the electrons to eject.
Based on the wave theory, the photoelectric effect should not be instantaneous. Wave energy can not be transferred to a particular electron but will be distributed to all the electrons present in the illuminated portion. Therefore, there must be a time lag between incidence of radiation and emission of electrons.
(b) (i) The slope (V0/v) of both the lines is the same which represents the universal constant known as Planck’s constant (h) = 6.62 x 10-34 JS
(ii) For the same frequency of incident radiations, M1 will be having greater kinetic energy as the value of V0 is greater for M1 material. It can be easily observed by drawing a vertical line, frequency being the same and intersecting M1 and M2 at different points.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments