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Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

Document Description: Electromagnetic Radiation: Wave & Particle Nature for JEE 2022 is part of Atomic Structure for Chemistry for JEE preparation. The notes and questions for Electromagnetic Radiation: Wave & Particle Nature have been prepared according to the JEE exam syllabus. Information about Electromagnetic Radiation: Wave & Particle Nature covers topics like What is Electromagnetic Radiation?, Parameters of Electromagnetic Radiations, Dual Behavior of Electromagnetic Radiation, Wave Nature of Light, Nature of Light, Particle Nature of Light, Emission & Absorption Spectrum, Photo-Electric Effect and Electromagnetic Radiation: Wave & Particle Nature Example, for JEE 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Electromagnetic Radiation: Wave & Particle Nature.

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Table of contents
What is Electromagnetic Radiation?
Parameters of Electromagnetic Radiations
Dual Behavior of Electromagnetic Radiation
Wave Nature of Light
Nature of Light
Particle Nature of Light
Emission & Absorption Spectrum
Photo-Electric Effect
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The dual character of the electromagnetic radiations was formulated through the Bohr’s Model of Atoms. The experimental data was presented for the same and the details will be mentioned as we go further.


What is Electromagnetic Radiation?

When electrically charged particles perform an accelerating motion, alternating electrical and magnetic fields are produced and transmitted. These fields traverse in the forms of waves known as electromagnetic radiation. A light wave is an example of electromagnetic radiation.

Electromagnetic radiation consists of two perpendicular waves, one electric and one magnetic, propagating at the speed of light (c). Electromagnetic radiation is radiant energy that includes radio waves, microwaves, visible light, x-rays, and gamma rays, which differ in their frequencies and wavelengths.

2D diagram of the EM Waves2D diagram of the EM Waves

  • Whenever a charge is placed in an electric or a magnetic field, it experiences a certain force acting on it or if multiple charges are placed, they experience an interaction due to another.
  • In the year 1870, James Maxwell became the first scientist to explain the interaction between the charges in the presence of the electric and magnetic fields.

Question for Electromagnetic Radiation: Wave & Particle Nature
Try yourself:Bohr Model of an atom could not account for?
View Solution

Properties of Electromagnetic Radiation

  • The oscillating charged particles produce oscillating electric and magnetic fields which are perpendicular to each other and both are perpendicular to the direction of propagation of the wave.
  • Unlike the sound and other waves in water, Electromagnetic waves do not require a medium i.e., they can travel in a vacuum too.
  • In vacuum all types of electromagnetic radiation, regardless of wavelength, travel at the same speed, i.e., 3.0 × 108 m s–1 (2.997925 × 108 m s–1, to be precise). This is called speed of light and is given the symbol ‘c‘. The frequency (ν ), wavelength (λ) and the velocity of the light is related to the equation:
    c = ν λ
    where:

    c = speed of light

    ν = frequency of the electromagnetic wave

    l = wavelength of the electromagnetic wave

  • Different kinds of units are used to represent electromagnetic radiation.

Parameters of Electromagnetic Radiations

  • Electromagnetic Radiation is characterized based on various properties like frequency (ν ) , wavelength(λ) , time period etc.
  • Apart from frequency and wavelength, some other parameters are also used to categorize electromagnetic radiation. One of these parameters is the wavenumber. ( ⊽). 
  • It is defined as the number of wavelengths per unit length. 
  • Its units are reciprocal of the wavelength⊽ unit, i.e., m–1. However commonly used unit is cm–1

EM Spectrum

Electromagnetic spectrum, the entire distribution of electromagnetic radiation according to frequency or wavelength. Although all electromagnetic waves travel at the speed of light in a vacuum, they do so at a wide range of frequencies, wavelengths, and photon energies.

Illustration of Electromagnetic SpectrumIllustration of Electromagnetic Spectrum

Question for Electromagnetic Radiation: Wave & Particle Nature
Try yourself:In what region(s) of the spectrum does this series occur?
(Visible region is from 360 to 780 nm)
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Dual Behavior of Electromagnetic Radiation

Electromagnetic Radiation is basically light, which is present in a rainbow or a double rainbow. It also is a spectrum consisting of radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays, and gamma rays. There are only two ways to transfer energy from one place to another place.

  1. Through a particle.
  2. Through a wave.

Electromagnetic radiation is a particle as well as a wave thus it is interesting to study its nature in quantum theory. Light is also electromagnetic radiation that contains frequencies.

Dual Nature of LightDual Nature of Light


Wave Nature of Light

A wave is a physical phenomenon characterized by its frequency, wavelength, and amplitude. In general, waves transfer energy from one location to another, in which case they have a velocity.

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE


Characteristics of Waves

  • Amplitude: Maximum displacement from the mean position it remains constant with distance except for stationary or standing waves.
  • Wavelength: It is the distance between two adjacent crest or troughs.
  • Frequency (m or n): The no of waves passing through a point in 1 second, unit-sec-1 or Hz.
  • Wavenumber: No. of waves present in the unit distance.Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE etc.
  • Velocity: linear distance traveled by wave in one second.Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE


Maxwell Electromagnetic Wave Theory (Wave Nature of Light) 

  • An accelerated electrically charged particle produces and transmits an electrical and magnetic field. These are transmitted in the form of waves known as electromagnetic waves or electromagnetic radiations.
  • He stated that light also possesses an electrical and magnetic field, and it is also known as electromagnetic radiations or e.m.w.Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE


Particle Nature of Light

  • The emission of free electrons from a metal surface when the light is shone on it, it is called the photoemission or the photoelectric effect. This effect led to the conclusion that light is made up of packets or quantum of energy. 
  • Now the question was whether the light quantum theory was indicative of the particle nature of light. Einstein already associated the light quantum with momentum. This strongly supported the particle nature of light and these particles were named photons.

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE


Planck's Quantum Theory 

  • He stated that a body radiates energy in the form of discontinuous energy packets or bundles. Each bundle of energy is known as quantum, and the quantum of light is known as photons. The energy of each quantum is directly proportional to the frequency of radiation.
  • Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE
    E = hv
    h = 6.62 × 10-34 Js.

Planck's Constant

  • Total energy absorbed or emitted by a body will be whole no. integral multiple of the energy of quantum. i.e., Eabs or Eemitted = nhν.

Example 1: Calculate the no. of photons emitted by the 60-watt bulb in 10 hrs. When the light of wavelength 6000 Å is emitted by it. 

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE = 6.5 × 1024 J

Question for Electromagnetic Radiation: Wave & Particle Nature
Try yourself:The number of photons of light of wavelength 7000 Å equivalent to 1 J are
View Solution

Energies in Electron Volts

  • Room temperature thermal energy of a molecule = 0.04 eV
  • Visible light photons = 1.5-3.5 eV
  • Energy for the dissociation of an NaCl molecule into Na  and Clions: = 4.2 eV
  • Ionization energy of atomic hydrogen = 13.6 eV
  • Approximate energy of an electron striking a color television screen (CRT display) = 20,000 eV
  • High energy diagnostic medical x-ray photons.= 200,000 eV (=0.2 MeV)

Typical Energies From Nuclear Decay:

  • gamma = 0-3 MeV
  • beta = 0-3 MeV
  • alpha = 2-10 MeV
  • Cosmic ray energies = 1 MeV - 1000 TeV
  • 1 MeV = 106 eV, 1 GeV = 109 eV, 1 TeV = 1012 eV.

Explanation of Black Body Radiations Using Planck's Quantum Theory

  • When a solid substance like the iron piece is heated, it emits radiation. As heating is continued, more and more energy is being absorbed by the atom, and hence, more energy will be emitted and therefore the energy of e.m.w. increases and frequency of e.m.w. increases and therefore, the body first becomes red then yellow, and finally white.
  • Therefore, it can be concluded that light posses a particle nature, and the energy of electromagnetic radiation depends upon frequency.

Explanation of Photoelectric Effect Using Planck's Quantum Theory

  • When a metal sheet is subjected to electromagnetic radiation of suitable frequency then some electrons are ejected from the metal surface, and these electrons are known as photoelectron, and the effect is known as the photoelectric effect.

Photoelectric effectPhotoelectric effect

  • If electromagnetic radiation of low frequency is used, then there is no ejection of electron despite the continuous increasing intensity. This observation was contradicting to Maxwell theory according to which energy electromagnetic radiation ∝ I but can be explained using Planck's quantum theory, i.e. E ∝ ν.

Emission & Absorption Spectrum

Spectra can be divided into two types based on absorption by gas or vapour & white light emission:

1. Emission Spectrum: Spectrum due to the emission of white light by gas at high temperature is known as an emission spectrum. This kind of spectrum usually consists of bright lines on the dark background. The emission of energy by electrons generates an emission spectrum.

2. Absorption Spectrum: The spectrum which occurs due to absorption of white light by gas and transmitted white light, it is termed as an absorption spectrum. Unlike the emission spectrum, it consists of dark lines on the bright background. it is due to the absorption of energy by electrons.

Spectra can be divided into two types depending on the spectral lines:

  • Line Spectrum or Atomic Spectrum: This is made up of distinct lines. When an electron in an atom excites and de-excites, this spectrum occurs. Emission & absorption spectra show the line spectrum.
  • Band Spectrum: It is a characteristic of a molecule. It consists of closely spaced lines called bands. In a  molecule, the vibration & rotation of atoms generates such a spectrum.

Photo-Electric Effect

Emission of an electron from the metal surface when the light of suitable frequency is subjected to the metal surface. The effect is known as the photoelectric effect and the ejected electrons are known as photoelectrons.

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

Terms Used in the Photoelectric Effect

1. Work function (w): It is the minimum amount of energy required to cause photoemission from the metal surface. It is also known as threshold energy or Binding energy. [Work function depends upon ionization energy and therefore w is minimum for alkali metals].
2. Threshold frequency (n0): The minimum value of frequency that can cause photoemissions. If n < n0, then there is no photoemission.
w = h nElectromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE
3. Threshold wavelength (l0): The maximum value of wavelength that can cause photoemission.
If l > l0, then photoemission is not possible.
4. Intensity (I): Energy falling on the metal surface of unit area of unit time
Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE
5. Photo intensity (IP): It is the number of photons falling per unit area per unit time.
Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE
Relation between I and Ip: I = Ip hn
Photo intensity is independent of frequency while intensity depends on frequency.
6. Power:  Total energy radiated per unit time.
Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE


Effect of Variation of Frequency

  • Effect of Photon Emission: I = Iphn
    If the frequency of subjected photon increases (intensity increases keeping photo intensity constant) then there is no change in the number of ejected photoelectrons as well as no change in photocurrent.

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

  • Effect on kinetic Energy: Average k. E. as well as K.E.max increases with an increase in frequency.
    hn - W = K Emax.
    K Emax = hn - hn0 (y = mx + c)
    K Emax = hn -Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE
    KEmax = hn - w 

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

  • Effect of Variation of Photo Intensity: On increasing intensity, keeping the frequency constant (i.e. increasing photo intensity) no of ejected photoelectrons increases as well as photocurrent increases.Graph of photocurrent vs voltage.
    Graph of photocurrent vs voltage.
  • Effect on Kinetic Energy: Average K.E. and K.Emax remain constant with change in photo intensity.

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

Stopping Potential or Retarding Potential (V0

It is the minimum potential required to stop the fastest moving electrons completely or it is the minimum potential at which photocurrent becomes zero.

eV0 = hn - w

eV0 = hn - hn

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

It can be commented that stopping potential increases with the increase in frequency however if photo intensity is changed there is no effect on stopping potential.

Graph of Photocurrent vs cell voltage. Graph of Photocurrent vs cell voltage. 

Example 2: In ultraviolet light of wavelength 280 nm is used in an experiment of photoelectric effect with lithium cathode (Work Function = 2.5 eV). Then calculate 

(i) K.Emax (ii) Stopping potential 

Electromagnetic Radiation: Wave & Particle Nature Notes | Study Chemistry for JEE - JEE

Question for Electromagnetic Radiation: Wave & Particle Nature
Try yourself:Which is not characteristic of Planck’s quantum theory of radiation?
View Solution

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