The Interaction of Electromagnetic Radiation with Matter

Emission and Absorption of Radiation
نعلم أن الذرة تكتسب طاقة وتفقدھا بصورة مستمرة وإن
انتقال الطاقة إلى الذرة يتم بواسطة طريقتين ھما:
Collisions with other atoms, and the transfer of kinetic
energy as a result of the collision. This kinetic energy is
transferred into internal energy of the atom.
Absorption and emission of electromagnetic radiation
وحيث أن عملية الليزر تعتمد على انتقال الطاقة من خلال امتصاص
الإشعاع الكھرومغناطيسي ثم تكبيره وانبعاثه Absorption
على شكل شعاع ليزر، لذا سندرس ظاھرة الامتصاص emission
والانبعاث.
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Emission and Absorption of Radiation
The interactions between electromagnetic radiation and
matter cause changes in the energy states of the electrons
in matter.
Electrons can be transferred from one energy level to
another, while absorbing or emitting a certain amount of
energy. This amount of energy is equal to the energy
difference between these two energy levels (E2-E1).
When this energy is absorbed or emitted in a form of
electromagnetic radiation, the energy difference between
these two energy levels (E2-E1) determines uniquely the
frequency (?) of the electromagnetic radiation:
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28/10/1431
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Example
The visible spectrum wavelength range is: 0.4 - 0.7
[?m] (400-700 [nm]).
The wavelength of the violet light is the
shortest, and the wavelength of the red light
is the longest. Calculate:
a) What is the frequency range of the visible
spectrum.
b) What is the amount of the photon’s energy
associated with the violet light, compared to the
photon energy of the red light.
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
Physics Academy
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Solution:
The frequency of violet light:
The frequency of red light:
The difference in frequencies:
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
28/10/1431
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The energy of a violet photon:
The energy of a red photon:
The difference in energies between the violet photon and the red
photon is:
2.15*10-19 [J]
This example shows how much more energy the violet photon
have compared to the red photon.
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
Physics Academy
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Question
Calculate in units of Nanometer, the
wavelength of light emitted by the transition
from energy level E3 to energy level E2 in
which:
E1 = 0 [eV]
E2 = 1.1 [eV]
E3 = 3.5 [eV]
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
28/10/1431
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Physics Academy
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Emission and Absorption of Radiation
Every system in nature "prefers" to be in the lowest energy
state. This state is called the Ground state.
When energy is applied to a system, The atoms in the material are
excited, and raised to a higher energy level.
(The terms "excited atoms", "excited states", and "excited
electrons" are used here with no distinction)
These electrons will remain in the excited state for a
certain period of time, and then will return to lower energy
states while emitting energy in the exact amount of the
difference between the energy levels (?E).
If this energy is transmitted as electromagnetic energy, it is called
photon.
Dr. Hazem Falah Sakeek www.physicsacademy.org & www.hazemsakeek.com
Physics Academy
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Spontaneous Emission
The emission of the individual photon is random, being done individually
by each excited atom, with no relation to photons emitted by other atoms.
When photons are randomly emitted from different atoms at different
times, the process is called Spontaneous Emission. Since this emission
is independent of external influence, there is no preferred direction for
different photons, and there is no phase relation between
photons emitted by different atoms.
Spontaneous emission is one of a family of processes, called
relaxation processes, by which the excited atoms return to
equilibrium (ground state).
This "classic" explanation assumes that the specific frequencies emitted by
an excited atom are the same