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<h2 class="hd hd-2 unit-title">Lasers: An Introduction</h2>
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<h3 class="hd hd-2">Lasers: An Introduction</h3>
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<h2 class="hd hd-2 unit-title">Population Inversion: Nd-YAG and Ruby Lasers</h2>
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<h3 class="hd hd-2">Population Inversion: Nd-YAG and Ruby Lasers</h3>
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<h2 class="hd hd-2 unit-title">Laser Cavity and Modes & Summary</h2>
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<h2 class="hd hd-2 unit-title">In-Lecture Question 6.2</h2>
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In-Lecture Question 6.2
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<p>Explain carefully why you need a system with at least three energy levels in order to create a laser.</p>
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