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Monochromatic light of wavelength 667 nm is produced by a helium neon laser. The power emitted is 9 mW. The number of photons arriving per sec on the average at a target irradiated by this beam is:

1. 3 × 10^16
2. 9 × 10^15
3. 3 × 10^19
4. 9 × 10^17

Correct answer: 3 × 10^16

Solution

The energy of one photon is given by E = hc/λ. Substituting h = 6.63 × 10^-34 J·s, c = 3 × 10^8 m/s, and λ = 667 × 10^-9 m, we get E ≈ 2.98 × 10^-19 J. The number of photons per second is then P/E = (9 × 10^-3 W) / (2.98 × 10^-19 J) ≈ 3 × 10^16 photons/s.

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