Скачать или смотреть Einstein Coefficients A and B| Absorption, Emission| Notes & Explanation| Engineering Physics| BSc

In this video, we derive the Einstein coefficients A and B step by step in a simple and clear manner, suitable for first-semester engineering and UG physics students.
First, we explain the physical meaning of Einstein coefficients:
B₁₂ for stimulated absorption
A₂₁ for spontaneous emission
B₂₁ for stimulated emission
Then, we calculate the number of absorption and emission processes per unit volume per unit time.
Next, we apply the thermal equilibrium condition, where the rate of absorption is equal to the total rate of emission (spontaneous + stimulated).
Using this equilibrium condition, we derive an expression for the energy density of photons.
This expression is then compared with Planck’s radiation formula.
Finally, we obtain the important relations between Einstein coefficients A and B, including:
The relation between A₂₁ and B₂₁
The relation between B₁₂ and B₂₁
This topic is very important for understanding:
Laser physics
Population inversion
Optical transitions
Quantum theory of radiation
The complete derivation is explained step by step, with clear logic and exam-oriented presentation.
📌 Perfect for:
BTech / BE / BSc Physics students | Laser Physics | Quantum Mechanics

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