محاضرة-2

Laser Principles:Lecturer-2-
ENERGY OF A PHOTON:
The "duality principle" of light states that "light cannot be described completely as either a particle or a wave, but has characteristics of both," i.e., there is a complementarily of the two concepts.
A photon is the smallest division of a light beam that retains the properties of the beam. The characteristics of a photon include its frequency, its wavelength, and its energy. A photon should not be visualized as a particle that has physical dimension or a specific location in space.
PLANCK S CONSTANT:
The energy of a photon is directly proportional to the frequency of the photon and is given by Equation 1.
Equation 1
E = h?
where: E = Energy of photon in joules (J).
? = Frequency of the photon in hertz.
h = Planck s constant = 6.625 × 10–34 joule-seconds.
Given: The frequency of a photon of HeNe laser light is 4.74 × 1014 Hz.
Find: The energy of the photon.
Solution: E = h?
E = (6.625 × 10–34 J.sec) (4.74 × 1014/sec)
E = 3.14 × 10–19 J






The frequency of a photon is determined by the relation described by

Equation 2.


where: c = The speed of light in vacuum, namely 3 × 108 m/sec.
? = Wavelength in meters.
Substitution of Equation 2 into Equation 1 yields Equation 3.
Equation 3


Given: The wavelength of a He-Ne laser light is 633 nm.
Find: The energy of the photon of this light.
Solution:



Given: A photon has an energy of 1.875 × 10–19 J.
Find: The frequency and the wavelength of the photon.
Solution: From Equation 1

From Equation 2—

ENERGY UNITS:
1- The joule is the basic energy unit in the International System of Units (sometimes referred to as the MKS system—meters, kilograms, seconds).
2-the electron volt (eV). It is defined as the amount of energy acquired by an electron, when it is accelerated by a potential difference of one volt, , 1eV = 1.602? 10-19J. The erg is a very small unit of energy. It takes ten million ergs to equal one joule of energy (1 erg=10-7J.)
The Wavenumber of a photon is the number of wavelengths of that photon contained in a length of one centimeter. The Wavenumber is given by
Equation 4.

where: k = Wavenumber in cm–1
? = Wavelength in cm.
The unit of Wavenumber is the reciprocal centimeter (cm–1).


Given: A photon has a wavelength of 514.5 nm.
Find: The Wavenumber of the photon.
Solution: Determine the wavelength in cm:






CONVERSION OF ENERGY UNITS:
TABLE 1. CONVERSION FACTORS FOR ENERGY UNITS
Unit joule electron
volt cm–1
joule 1 6.24 × 1018 5.034 × 1022
electron
volt 1.602 × 10–19 1 8065.73
cm–1 1.9864 × 10–23 1.24 × 10–4 1

H.W

Given: A He-Ne laser photon has a wavelength of 632.8 nanometers
Find: (a) Photon energy in joules
(b) Photon energy in cm–1

















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