#### Lasers-Einstein A and B coefficients;Ruby and He-Ne lasers; Characteristics of laser light-spatial and temporal coherence; Focusing of laser beams; Three-level scheme for laser operation; Holography and simple applications.

Questions:

**Q 1. **In He-Ne laser, what is the function of He gas? Explain the answer with the help of energy level diagram for He-Ne laser ? **[15 marks]**

**Q 2. **Explain the principle of (i) induced absorption, (ii) spontaneous emission and (iii) stimulated emission . Show that the ratio of Einstein’s coefficients is given by $\frac AB=\frac {8\pi h \nu ^3}{c^3}$ **[20 marks]
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Video Solution at 11 pm