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Monochromatic light of frequency 6.0 × 10^14 Hz is produced by a laser. The power emitted is 2 × 10^-3 W. The number of photons emitted, on the average, by the sources per second is:

1. 5 × 10^16
2. 5 × 10^17
3. 5 × 10^14
4. 5 × 10^15

Correct answer: 5 × 10^16

Solution

The energy of one photon is given by E = hν, where h = 6.63 × 10^-34 J·s and ν = 6.0 × 10^14 Hz. Thus, E = 6.63 × 10^-34 × 6.0 × 10^14 = 3.978 × 10^-19 J. The number of photons emitted per second is Power/Energy per photon = (2 × 10^-3)/(3.978 × 10^-19) ≈ 5 × 10^16.

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