The approximate number of photons in a femtosecond (10−15 s) pulse of 600 nanometers wavelength light from a 10-kilowatt peak-power dye laser is
A. 103
B. 107
C. 1011
D. 1015
E. 1018
(GR9677 #59)
Solution:
Number of photon, n = Etotal / Esingle
Energy of a single photon, Esingle = hc/λ
h = 6.63 × 10−34 Js
c = 3 × 108 m/s
λ = 600 nanometers = 600 × 10−9 m
→ Esingle = 3 × 10−19 J
From Power, P = W/t = Etotal/t
→ Total energy, Etotal = Pt
P = 10 kW = 104 W
t = 10−15 s
→ Etotal = 10−11 J
n = Etotal / Esingle = 10−11 / (3 × 10−19) = 0.3 × 108 = 3 × 107 J
Answer: B
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