# CBSE Class 12 Physics Revision Notes Chapter 11

## Class 12 Physics Chapter 11 Notes

Dual Nature Of Radiation And Matter, the Class 12 Physics Chapter 11 notes cover a brief outline of all important concepts. Students can understand basic equations, essential derivations and important questions included in the NCERT books. This Chapter is of utmost importance for the CBSE examinations. It helps students to gain knowledge of the fundamental topics such as the photoelectric effect, its laws, electron emission, etc., and solve various numerical problems based on the formulas.

Visit the Extramarks web portal and mobile app and get access to all the detailed and well-explained Class 12 Physics Chapter 11 notes.

### Key Topics Covered In Class 12 Physics Chapter 11 Notes

Topics explained in the Class 12 Physics Chapter 11 notes are:

• Photoelectric Effect
• Experimental study of Photoelectric Effect
• Einstein’s Photoelectric Equation
• Observations of Hertz and Lenard
• Matter Waves
• Wave Nature of particles
• de Broglie’s Relation

A brief of the topics covered in Class 12 Physics Chapter 11 notes is as under.

Introduction:

In the Class 12 Physics Chapter 11 Notes, students learn about various phenomena such as interference, polarisation and diffraction of light with the help of wave nature. Furthermore, Maxwell’s equations, the Photoelectric effect by Hertz and Lenard, de Broglie’s relation, etc., are explained in detail in the chapter  Dual nature of radiation and matter. . Some basic equations, essential derivations and important questions are also included in the notes of Class 12 Physics Chapter 11.

Cathode Rays

Cathode rays are defined as the stream of fast-moving electrons of an atom. They can be produced in a discharge tube at a low pressure below 0.001 mm of a mercury column.

Properties:

• These are not electromagnetic rays.
• These rays are deflected by the electric and magnetic fields.
• When the cathode rays fall on the metals, they produce heat.
• Without the need for puncturing, the cathode rays can easily pass through the thin aluminium or gold foils.
• The cathode rays can undergo any physical and chemical change.
• These rays travel in a straight line with high velocity and energy and have the ability to cast shadows on objects.
• They can produce X-rays by striking a target having high atomic weight.
• They affect the photographic plate as the cathode rays produce fluorescence and phosphorescence in particular substances.
• The specific charge of an electron = 1.7589 x 1011 C / kg.
• Millikan’s oil drop experiment helped in measuring the charge of an electron, which was experimentally found to be 1.602 x 10-19 C.

Positive Rays

The scientist Goldstein discovered positive rays. These are defined as the moving positive ions of gas loaded in the discharge tube. The mass of these particles is equal to the mass of the atoms of the gas.

Properties:

• The positive rays consist of fast-moving positively charged particles.
• These rays get deflected in the magnetic fields and electric fields.
• These rays can only travel in a straight line.
• The speed of the positive rays is less than the speed of the cathode rays.
• The positive rays have the ability to produce fluorescence and phosphorescence.

Electron Emission:

The minimum energy that an electron requires to escape the surface of a metal is known as the work function of that metal. It is denoted by o and is measured in electron volt (Symbol- eV).

One electron volt(eV) is defined as the energy that the electron gains for accelerating by a potential difference of 1 volt. We can say that 1 eV = 1.602 ×  10-19 J.

The value of the work function is dependent on the properties of the metal and the nature of its surface.

The process of emission of electrons from the surface of any metal is known as electron emission. It is obtained from the following three processes.

• Thermionic: By heating, sufficient thermal energy is imparted to the free electrons so that they can escape from the metal surface.
• Photoelectric emission: When the light of a suitable frequency is illuminated on a metal surface, the electrons of the surface are emitted. These electrons are called photoelectrons.
• Field emission: By applying an electric field to the metal, electrons can be pulled out of the metal.

Photoelectric Effect

Hertz’s observations

In 1887, Heinrich Hertz discovered the phenomenon of photoelectric emission during his experiments on electromagnetic waves. He observed that when light strikes across a detector loop (metal surface), the electrons on the surface acquire sufficient energy and overcome the attraction force of the positive ions to escape into the surrounding space.

Observation by Hallwach and Lenard

The scientists Hallwachs and Lenard discovered through their experiments that there exists a minimum cut-off frequency below which no electrons can be emitted. This frequency is regarded as the threshold frequency.

It was observed that certain metals like zinc(Zn), cadmium(Cd), magnesium(Mg), etc., respond to UV light of short wavelength to cause the emission of electrons from the surface. Some alkali metals, namely Lithium(Li), sodium(Na), potassium(P), caesium(Cs) and rubidium(Rb) are sensitive even to  visible light.

Students may refer to the Class 12 Physics Chapter 11 notes to get a deeper understanding of these concepts.

Photoelectric Effect- Experimental Study

Experimental setup: Photosensitive plate (emitter), metal plate (collector) attached in an evacuated glass/quartz tube. This tube will allow the electrons to flow from the emitter to the collector in the absence of any air resistance. The battery is used to maintain the potential difference(V) between the two plates. The polarity can be reversed by the commutator.

The photosensitive plate, i.e., the emitter, is used to absorb the visible light and emit electrons. The metal plate (collector) is used to receive all the electrons that are released by the emitter. This forms a photoelectric current flow  (from emitter to collector). This is always opposite to the directions of the electrons. The potential difference(V) and current (I) between the plates can be measured using a voltmeter and ammeter, respectively.

1. Effect of intensity

In one second, the number of photoelectrons emitted is directly proportional to the photocurrent, where the frequency is fixed. It means that the rate at which photoelectrons are emitted is proportional to the intensity of incident energy.

1. Effect of potential

The stopping potential of the incident radiation is independent of the intensity for a fixed frequency. We can say that the maximum kinetic energy of the photoelectrons is dependent on the light source and the nature of the photosensitive emitter plate. It is independent of the intensity of the incident radiation.

The variation of stopping potential for a particular frequency of incident radiation is the least negative or retarding potential Vo applied to the emitter plate for which the photocurrent becomes zero and ends.

1. Effect of the frequency:

It means that the higher the frequency of incident radiation, the higher will be the maximum kinetic energy of the photoelectrons. Therefore, we will need more retarding power to stop them. The graph depicts the variation of the photoelectric current with collector plate potential for different frequencies of incident radiation.

Consider a photosensitive material. The variation of stopping potential, i.e., Vo, with the frequency of incident radiation is shown in the graph.

Laws of Photoelectric Effect

• Photoelectric current (e-) is directly proportional to the intensity of incident radiation for specific photosensitive material and the frequency of incident light which is above the threshold frequency.
• For a certain photosensitive material and the frequency of the incident radiation, the saturation current is defined to be directly proportional to the intensity of incident radiation, but the stopping potential is independent of the intensity.
• There exists a minimum cut-off frequency below which no electrons can be emitted. This frequency is regarded as the threshold frequency.
• The maximum kinetic energy of the emitted photoelectrons is unaffected by its intensity but varies linearly with the frequency of the incident radiation over the threshold frequency.
• Even when the incident radiation is made extremely dim, the emission of photoelectrons is an instantaneous process with no apparent time lag.

Photoelectric Effect and the Wave Theory of Light

The concepts of interference, diffraction, and polarisation are well explained using the wave theory of light.

However, the experiments based on the photoelectric effect cannot be described using this theory.

The wave theory model fails to describe the most prominent characteristics of the emission of photoelectrons. Hence, the phenomenon of the photoelectric effect is explained with the help of a new hypothesis known as the photon picture of light.

Einstein’s Photoelectric Equation:

Albert Einstein proposed that the radiation energy is built of discrete units. He stated that the photoelectric emission does not occur by steady absorption of energy from radiation. He named these discrete units the quanta of energy of radiation. Each quantum of energy is denoted by h, where is the frequency of light and h is Planck’s constant.

Therefore, Einstein’s photoelectric equation is given as

Ek =ho or

Ek =hho or

Ek =h(o)

Particle nature of light:

The photoelectric effect gives evidence that light in interaction with matter behaves as if it was made of quanta (packets of energy), each of energy h. The particles had some definite value of energy and momentum. This particle was found to be a photon.

1. The incident radiation behaves if it is made up of photons.
2. Each photon has energy E=h and momentum p (= h/c), where c is the speed of light.
3. Photons are electrically neutral. They are not deflected by any electric fields or magnetic fields.
4. In a photon-particle collision, the total energy and total momentum remain unchanged.

Wave nature of Matter:

It was proposed by de Broglie. The wavelength is given as

= hp=hm where p is the momentum.

Davisson-Germer Experiment:

The de Broglie’s wave nature of the material particle was confirmed by Davisson and Germer in 1927 and then by GP Thomson in 1928. Ni crystal was used to verify this experiment. Davisson and Germer found that the intensity of the scattered beam of electrons is different at different scattering angles. It is maximum for a 50° diffraction  angle at a potential difference of 54 V. the wavelength is given as

= 12.27VA

The Photoelectric Cell

The working of the Photoelectric cell is based on the photoelectric effect. The device’s electrical properties get influenced by light. It is also known as the electric eye. They are of three types: Photoemissive cell, Photovoltaic cell and Photoconductive cell.

The Photocells are employed in the television camera for scanning and telecasting scenes and in the reproduction of sound in motion pictures. In the manufacturing industry, photoelectric cells are used  to detect  microscopic flaws  in the metal sheets.

Students may consider studying using the Extramarks Chapter 11 Physics Class 12 notes to enhance their learning experience!

### Class 12 Physics Chapter 11 Notes: Exercises &  Solutions

The Class 12 Physics Chapter 11 notes are important to gain in-depth knowledge of all concepts included in this chapter. The solutions to exercise problems and key points included in the notes help students with a quick revision. With the help of these Class 12 Physics Chapter 11 notes, students can keep themselves updated about the weightage and marking system of each chapter. If they face any doubts or queries, the students can go through the notes provided by Extramarks. Along with the board examinations, students can also prepare themselves for various entrance exams such as IIT, JEE, NEET, etc. The Class 12 Physics Chapter 11 notes focus on strengthening the fundamental concepts and core knowledge.

Different academic notes are provided on the Extramarks web portal. The CBSE Class 12 Physics Chapter 11 notes will help students to ease their burden to attain high scores. They may consider solving various CBSE sample papers for efficient and productive preparation. To achieve expertise, students are advised to solve all CBSE extra questions included in the CBSE revision notes.

### Key Features Of Class 12 Physics Chapter 11 Notes

The key features of Class 12 Physics Chapter 11 notes compiled by Extramarks include:

• It is a great resource for planned exam preparation.
• It follows the latest CBSE Syllabus.
• It is prepared by some experienced faculty to ensure the best quality.
• It can be accessed on any electronic device such as mobile, tab, etc.
• It ensures that every student understands each concept clearly.