Question

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

(a) 4.9 X 10^15

(b) 5.9 X 10^15

(c) 6.9 X 10^15

(d) 7.9 X 10^15

The question was posed to me in an interview for job.

The question is from Helium Neon Laser in chapter Laser of Engineering Physics II

Answer 1

Right option is (b) 5.9 X 10^15

Explanation: We know, for a He-Ne laser, λ = 6328 Å = 6.328 X 10^-7 m

Power = 2.5 mW = 2.5 X 10^-3 E

Energy = Power X Time = 0.0025 X 1

= 0.0025

Number of photons in each pulse = Energy X λ / hc

= 0.0025 X 6.6 X 10^-7/6.6 X 10^-34 X 3 X 10^8

= 7.9 X 10^15.