Interaction of Electromagnetic Radiation with Matter Lecture on Interaction of Electromagnetic Radiation with Matter By Dr. Pragati Kumar Department of Nano Sciences and Materials Central University of Jammu

Electromagnetic Radiation Examples: Light Waves X-rays γ-rays Microwaves etc.

Interaction of Electromagnetic Waves with Matter Particles EM waves may interact with matter particle in different ways, the most important interactions are; EM waves may give entire energy to matter particle. (Photoelectric Effect) EM waves may give a part of their energy to matter particles. (Compton Scattering) EM waves may materialize into electron and positron. (Pair Production)

Photoelectric Effect A υ e-s Incident photon of Photoelectric Effect Metallic Plate B A Anode Vacuum Chamber e-s Heinrich Hertz performed the experiments in 1887 and found ejection of electrons from the surface of metal due to impinging of photons of sufficient frequency.

Effect of Anode Voltage: υ Incident photon of Effect of Anode Voltage: A A

Effect of Intensity: Intensity i Effect of Intensity: Intensity i.e number of photons falling on per unit area per unit time. Metallic Plate A Anode Vacuum Chamber

A Effect of Energy or frequency of photons: Metallic Plate Vacuum Chamber A

 i Cs K Na

Laws of Photoelectric Effect:- There is no time lag between incident radiation (photon) and ejected photoelectron. The rate of photo-emission is directly proportional to intensity of incident radiation (light). The velocity and hence the kinetic energy of photo-electrons is independent of intensity of incident light. The velocity and hence the kinetic energy of photo-electrons is directly proportional to frequency of incident radiation. The emission of electron take place above a certain frequency known as threshold frequency. This frequency is characteristic frequency of photo-metal used.

Failures of Classical theory: According to classical theory: Intensity of a wave is the energy incident per unit area per unit time and given by . Where n & a are frequency and amplitude of wave. Higher intensity of light, greater the energies of the photoelectrons. Classical wave theory cannot explain the first 3 observations of photoelectric effect. Existence of the threshold frequency. Almost immediate emission of photoelectrons as the energy of em waves spread across the wavefronts, a period of time should be lapsed before an electron accumulates enough energy to leave the metal. Higher frequencyof light, greater the energies of the electrons.

Quantum Theory of Photoelectric Effect On the bases of Planck’s quantum which predicts that emission of radiation takes place in small packets of energy, known as quanta or photon or bundle, Einstein argued that light is not only emitted in quanta but travels as quanta i.e the photons preserve their identity throughout their life. He postulated that during interaction of photon with matter, either it may give its entire energy to material’s electron or none at all. When photon gives its energy to electron A part of this energy is used to overcome the binding forces of nucleus, the used energy for above is known as work function of that metal. Remaining energy is used to impart kinetic energy to same electron. If ν is the frequency of incident photon and v be the velocity of photoelectron then h ν=W+mv2/2 Where W is work function of metal.

Compton Scattering λ` λ Scattered photon of λ Incident photon of Scattering of x-rays by electron in rest with wavelength larger than the wavelength of incident x-rays along with original wavelength.

Compton's experimental results: or Unmodified radiation Scattered Modified radiation Relative Intensity

Failures of Classical theory: According to classical theory Frequency of radiation is characteristic of wave irrespective to medium. Classical wave theory cannot explain the observation Change in frequency of radiation as oscillating electric field vector in the incident wave of frequency υ acts on the free electrons in the scattering target and sets them oscillating at that same frequency. These oscillating electrons, like charges surging back and forth in a small radio transmitting antenna, radiate electromagnetic waves that again have this same frequency υ.

Quantum Theory of Compton Scattering Compton (and independently Debye) interpreted experimental results by postulating x-ray beam as collection of photons of frequency υ instead of wave, each of photon having energy hv. Interaction of photons-electrons results the in emerging of "recoil" photons from the target make up the scattered radiation. During interaction the incident photon transfers some of its energy to the electron with which it collides and hence the scattered photon must have a lower energy hυ‘

Compton Shift: Quantitatively Compton shift can be calculated using conservation laws;- Energy conservation law where m= on squaring above equation we have Momentum conservation law

Pair Production An excellent example of the conversion of radiant energy into rest mass energy as well as into kinetic energy. e- γ photon e+ Creating an electron and a positron (the pair) and endowing them with kinetic energies as a consequence of high energy photon’s interaction with a nucleus.

The phenomena of pair production does not occurs in empty space because of violation of conservation laws of momentum and energy as enough momentum for the process is carried away and negligible fraction of energy is absorb by nucleus due to its enormous mass. Minimum, or threshold, energy needed by a photon to create a pair is 2m0c2 or 1.02 MeV (1MeV =106 eV), which is a wavelength of 0.012 Ȧ.

Relative Probability of the Photoelectric effect, Compton Scattering and pair production as a function of energy of radiation. In a light element (Carbon) In a heavy element (Lead)

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