Physics Ph 1229 - KUET

Md. Kamrul Hasan Reza

Department of Physics

Khulna University of Engineering & Technology Khulna-9203, Bangladesh

Tel.: +880-41-769468~75 Ext. 587(O), 588 (R)

e-mail: mkhreza@phy.kuet.ac.bd, mkhreza1@gmail.com Website : www.kuet.ac.bd/phy/reza/

Instagram: mkhreza1@ Md. Kamrul Hasan Reza

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Welcome to my Class

Physics Ph 1229

11:45 AM

March 10, 2021

COVID-19 Precautions

Don’t be afraid

Be aware of the pandemic

Use appropriate outfits if you compelled to go out

Try to maintain proper diet

Do not forget to exercise (at least one hour) regularly

Try to follow the guidelines of WHO and Bangladesh Government

 Try to stay at home

LASER

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser"

originated as an acronym for "light amplification by stimulated emission of radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laborarories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

The Ruby LASER

The Ruby LASER

Einstein’s A and B Coefficients

An atom in the excited state emits radiations. This is possible in two ways:

Spontaneous emission and Stimulated emission

The emitted radiation has the same frequency, phase and polarization as the inducing radiation and propagate in the same direction

Einstein predicted it in 1917 purely from thermodynamical considerations.

Consider an atom in two states. Let the number of atoms per unit volume per unit time in state 1 be N₁ and in state 2, N₂.

Let ψ represents the energy density of radiation for frequency ν.

Then ψdν represents the energy density between frequency range ν and ν+dν.

Assuming that the rate of absorption of radiation of frequency ν from state 1 to state 2 is proportional to the radiation energy density ψ, the number of absorptions per unit volume per unit time will be given by

𝑁₂𝐴₂₁

𝑁₂𝐵₂₁𝜓

Here B₁₂ is a constant of proportionality from state 1 to state 2.

The number of spontaneous emissions per unit volume per unit time will be given by

The number of stimulated emissions per unit volume per unit time depends upon N₂ and ψ.

Therefore the number of stimulated emissions per unit volume per unit time is given by

𝑁₂𝐵₂₁𝜓

𝑁₁𝐵₁₂𝜓 = 𝑁₂𝐴₂₁ + 𝑁₂𝐵₂₁𝜓 When thermal equilibrium is reached,

(1)

𝑜𝑟 𝜓 = 𝑁₂𝐴₂₁

𝑁₁𝐵₁₂ − 𝑁₂𝐵₂₁

𝜓 = 𝐴₂₁

(𝑁₁/𝑁₂)𝐵₁₂ − 𝐵₂₁

According to Boltzmann’s law, at thermal equilibrium 𝑁₂ = 𝑁₁𝑒^{− 𝐸2𝑘𝑇−𝐸1}

or ^𝑁1

𝑁₂ = 𝑒 ^{𝐸2−𝐸1𝑘𝑇}

(2)

(3)

Substituting this value in eqn. (2) 𝜓 = 𝐴₂₁

𝑒 ^{𝐸2𝑘𝑇−𝐸1} 𝐵₁₂ − 𝐵₂₁

𝜓 = 8𝜋ℎ𝜐³ 𝑐³

1

𝑒^{ℎ𝜐𝑘𝑇} − 1 According to Planck’s law of radiation

(5) (4)

Taking E₂ - E₁= hν and rearranging eqn. (4), we get 𝜓 = 𝐴₂₁

𝐵₁₂

1

𝑒 ^{𝐸2𝑘𝑇−𝐸1} − 𝐵𝐵²¹₁₂

Comparing eqns. (5) and (6) 𝐵₂₁

𝐵₁₂ = 1

𝑜𝑟 𝐵₂₁ = 𝐵₁₂ (7) (6)

𝑎𝑛𝑑 𝐴₂₁

𝐵₁₂ = 8𝜋ℎ𝜐³ 𝑐³

𝑜𝑟 𝐴₂₁ = 𝐵₁₂ 8𝜋ℎ𝜐³ 𝑐³

𝑜𝑟 𝐴₂₁ = 𝐵₂₁ 8𝜋ℎ𝜐³ 𝑐³

(8)

(9)

Eqns. (8) and (9) are Einstein’s equations. A₂₁, B₁₂ and B₂₁ are called Einstein’s coefficients. These coefficients are related to each other.

The Helium-Neon LASER

The Helium-Neon LASER

In dye lasers the laser medium is a liquid containing organic dye molecules that can emit light over a range of wavelengths; adjusting the laser cavity changes, or tunes, the output wavelength.

Chemical lasers are gas lasers in which a chemical reaction generates the excited molecules that produce stimulated emission.

Application of LASER

Laser beams can travel long distances without dispersion and can carry vast amounts of energy

Manufacturing industry: cutting, drilling, soldering etc.

Medical Purpose: surgery, treatment of human and animal cancers.

Long distance data transmission Military and Research