The number of photons emitted for a 2.5 mW He-Ne laser is ________.

  1. 4.9 × 1015
  2. 5.9 × 1015
  3. 6.9 × 1015
  4. 7.9 × 1015

Answer (Detailed Solution Below)

Option 4 : 7.9 × 1015
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Detailed Solution

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For a He-Ne laser:

λ = 6328 Ȧ = 6.328 × 10-7 m

Calculation:

Power = 2.5 mW = 2.5 × 10-3 W

Energy = Power × Time = 0.0025 × 1 = 0.0025

The number of photons in each pulse will be:

\(n=Energy × \frac{λ}{ hc}\)

\(n=0.0025× \frac{6.328 × 10^{-7}}{ 6.6 × 10^{-34} × 3 × 10^8}\)......( λ  = 6.328 × 10-7)

n = 7.989 × 1015

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