Dual nature of radiation and matter Class 12th Notes - Free NCERT Class 12 Physics Chapter 11 Notes - Download PDF

The NCERT Class 12 Physics chapter 11 notes covers a brief outline of the chapter Dual nature of radiation and matter. Electron Emission, Methods of Electron Emission, Photoelectric Effect, Hertz's Observation, Hallwachs' and Lenard's Observations, Effect of Intensity of Light on Photocurrent, Effect of Potential on Photoelectric Current, Effect of Frequency of Incident Radiation on Stopping Potential, Laws of Photoelectric Effect, and many other topics are covered in the Dual Nature of Radiation and Matter class 12 notes.

The chapter's basic equations are also addressed in Class 12 Physics chapter 11 notes. All of these subjects are covered in the Dual Nature of Radiation and Matter Class 12 notes pdf download. The essential derivations are not covered in the CBSE physics chapter 11 notes for Class 12.

Also, students can refer,

• NCERT Notes Class 12 Physics
• NCERT Solutions for Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter
• NCERT Exemplar for Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter

NCERT Class 12 Physics Chapter 11 Notes

Electron Emission
The attraction forces of the ions keep free electrons inside the metal surface; the electron can only get out if it has enough energy to overcome the attractive pull.

An electron must be supplied a certain amount of energy in order to be pulled away from the metal's surface.

The work function of a metal is the minimum energy required for an electron to escape from its metal surface.

Methods of Electron Emission

Thermionic emission:-

The free electrons can be given enough thermal energy to allow them to come out of the metal by adding heat.

Field emission:-

Electrons can be drawn out of metal by applying a very powerful electric field (of the order of 108 V m–1), as in a spark plug.

Photo-electric emission:-

Electrons are emitted when an appropriate frequency of light strikes a metal surface.

Photoelectrons are electrons that are created by light.

Photoelectric Effect

Electrons are emitted from metallic surfaces when they are irradiated by electromagnetic radiation.

The photoelectric effect is the name for this phenomenon.

The electrons in the metal escape the pull of ions in the metal by absorbing energy from incident electromagnetic radiation.

Hertz’s Observation

Heinrich Hertz discovered in 1887 that when light strikes a metal surface, some electrons close to the surface acquire enough energy from the incident radiation to overcome the pull of the positive ions in the surface's substance.

The electrons escape from the metal's surface into the surrounding space after obtaining enough energy from the incoming light.

Hallwachs’ and Lenard’s Observations

Hallwachs and Lenard discovered that there is a minimum frequency, known as the threshold frequency, below which no electrons are emitted, through a series of tests.

Experimental Study of Photoelectric Effect

The following items make up the experimental setup:

The photosensitive plate (emitter) and the metal plate (collector) in an evacuated tube allow electrons to flow easily from emitter to collector without encountering any air resistance.

To absorb visible light and emit electrons, a photosensitive plate (emitter) is used.
A metal plate (collector) receives electrons released by the emitter, forming a photoelectric current flow from the collector to the emitter plate (opposite to the flow of electrons)

Short-wavelength monochromatic light (meaning high frequency)

Through a potential difference, a battery is used to accelerate emitted electrons.

Photoelectric current flow causes a potential difference between the emitter and collector plates, which can be measured with a voltmeter.

Photoelectric current is measured with an ammeter.

Effect of Intensity of Light on Photocurrent

The number of photoelectrons emitted per second is directly proportional to the photocurrent.

This means that the rate at which photoelectrons are emitted is proportional to the intensity of incident energy.

Effect of Potential on Photoelectric Current

The stopping potential of incident radiation is independent of its intensity for a given frequency.

In other words, photoelectrons' maximum kinetic energy is dependent on the light source and the material of the emitter plate, but not on the intensity of incident radiation.

The cut-off or stopping potential in the above figure for a certain frequency of incident radiation is the least negative (retarding) potential V₀ applied to plate A for which the photocurrent ends or becomes zero.

Effect of Frequency of Incident Radiation on Stopping Potential

This means that the higher the frequency of incident light, the higher the photoelectrons' maximum kinetic energy.

As a result, we'll need more retarding power to completely stop them.

The graphic above depicts the variation of photoelectric current with collector plate potential for various frequencies of incoming radiation.

For a given photosensitive material, the variation of stopping potential V0 with incident radiation frequency v is shown above.

Laws of Photoelectric Effect

(i) The photoelectric current is exactly proportional to the intensity of incident light for a particular photosensitive material and frequency of incident radiation (above the threshold frequency).
(ii) For a particular photosensitive material and incident radiation frequency, saturation current is found to be proportional to incident radiation intensity, whereas stopping potential is independent of incoming radiation intensity.

(iii) There is a minimum cut-off frequency of incident radiation for a specific photosensitive material, known as the threshold frequency, below which no photoelectrons are generated, regardless of how bright the incident light is.

The stopping potential, or equivalently the maximum kinetic energy of the emitted photoelectrons, grows linearly with the frequency of the incident radiation over the threshold frequency but is unaffected by its intensity.

(iv) Even when the incident light is made extremely weak, photoelectric emission is an instantaneous process with no apparent time lag (10^{– 9}s or less).

Photoelectric Effect and Wave Theory of Light

The wave picture of light was used to explain interference, diffraction, and polarisation.

The experimental investigation of the photoelectric effect, on the other hand, cannot be described using the wave theory of light.

The wave model fails to describe the most fundamental characteristics of photoelectric emission.

As a result, a new hypothesis called the photon picture of light was presented to explain the photoelectric effect.

Photon

Photons are energy packets or quanta connected with electromagnetic radiation.

E = h v gives the energy of a photon, where v is the frequency associated with the photon and h is Planck's constant.

Einstein’s Photoelectric Equation: Energy Quantum of Radiation

To explain the photoelectric effect, Albert Einstein presented a radical new model of electromagnetic radiation (quanta of energy of radiation) in 1905. The energy of each quantum of radiant energy is hv, where h is Planck's constant and v is the light frequency.
An electron absorbs a quantum of energy (hv) of radiation in the photoelectric effect.

If the amount of energy absorbed is more than the minimal amount of energy required for the electron to escape from the metal surface, the electron is expelled with the maximal kinetic energy given by

The value of the work function for a given material is constant and is determined by the substance's nature.

Threshold frequency

For a given photosensitive material, there is a minimum cut off frequency vo for which the stopping potential is zero; this minimum cut off frequency vo is referred to as threshold frequency.

Intensity of Light

The amount of photons present in light determines its intensity.

The frequency of incident light has no bearing on it.

Kinetic Energy of Photoelectron

As or .the photoelectron's kinetic energy is proportional to the frequency of incident light.

It is unaffected by the intensity of the radiation.

Number of Photoelectrons Emitted and Frequency of Incident Light

The photoelectrons emitted in the photoelectric effect are solely dependent on the intensity of light.

If v > vo, it is independent of the frequency of incident light.

Photoelectric Current

Photoelectric Current is the rate at which photoelectrons are emitted from a metal surface.It has a direct relationship with the amount of incident radiation.

Photoelectric Cell

A photocell is a device that uses the photoelectric effect in a technical way.

It's an electronic device whose electrical properties are influenced by light.

It's also referred to as an electric eye.

Photocells are employed in the television camera for scanning and telecasting scenes, as well as in the reproduction of sound in motion pictures.

They're utilised in the manufacturing industry to detect small faults or holes in metal sheets.

Wave Nature of Matter

Interference, diffraction, and polarisation are all examples of light's wave nature.

Radiation, on the other hand, behaves as if it were made up of a bunch of particles – the photons – in the photoelectric and Compton effects, which involve energy and momentum transfer.

A logical question arises: If radiation has a dual (wave-particle) nature, don't natural particles (electrons, protons, etc.) have wave-like properties as well?
Matter, according to the de Broglie hypothesis, has a wave-like quality.

If radiation has two sides, then matter should as well.

The wave length associated with a particle with momentum p is determined by = h/p = h/(m v), where m is the particle's mass and v its speed, according to De Broglie.
The de Broglie relation is known as the de Broglie wavelength, and the above equation is known as the de Broglie relation. The preceding equation indicates that for a heavier (large m) or more energetic particle, is smaller (large v).

Davisson and Germer’s Experiment

Davisson and Germer's experiment provides experimental proof of the concept of material particle-wave nature.

Davisson and Germer's experimental setup is schematically depicted in the diagram below:

An electron gun was utilised to create a beam of electrons in the experiment.

This tiny beam of accelerated electrons was directed into a nickel crystal, where the crystal's atoms scattered the electrons in various ways.

A detector measured the intensity of the electron beam dispersed in a certain direction.

On a circular scale, this detector was designed to rotate.

As a result of the experiment, scientists examined the intensity of scattered electron beams at various latitude angles or scattering angles, which are the angles between the incident and scattered electron beams. The accelerating voltage was varied from 44 to 68 V during the experiment.

For an accelerating voltage of 54V and a scattering angle of θ=500, a high peak in the intensity (I) of the scattered electron was observed.

The constructive interference of electrons scattered from distinct layers of the regularly spaced atoms of the crystals causes the peak to appear in a specific direction.

The wavelength of matter waves was determined by electron diffraction studies to be 0.165 nm.

Significance of NCERT Class 12 Physics Chapter 11 Notes

The dual nature of radiation and matter Class 12 notes will help you review the chapter and obtain a better understanding of the main issues addressed.

This NCERT Class 12 Physics chapter 11 notes are also beneficial for competitive exams such as VITEEE, BITSAT, JEE Core, NEET, and others, as they cover the main themes of the CBSE Class 12 Physics Syllabus.

Class 12 Physics chapter 11 notes pdf download can be utilised for offline preparation.

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