Answer:
[tex]3.3\cdot 10^{15}[/tex]
Explanation:
First of all, let's calculate the energy of a single photon of wavelength
[tex]\lambda=550 nm=5.5\cdot 10^{-7}m[/tex]
which is given by
[tex]E_1 = \frac{hc}{\lambda}=\frac{(6.63\cdot 10^{-34}Js)(3\cdot 10^8 m/s)}{5.5\cdot 10^{-7} m}=3.6\cdot 10^{-19} J[/tex]
The power of the flash is
[tex]P=1.2 mW=0.0012 W[/tex]
and the time it lasts is
[tex]t=100 ms=0.1 s[/tex]
so the total energy delivered in one flash is
[tex]E=Pt=(0.0012 W)(0.1 s)=1.2\cdot 10^{-3}J[/tex]
This energy contains exactly N photons each of energy [tex]E_1[/tex], so the number of photons emitted in one flash is
[tex]N=\frac{E}{E_1}=\frac{1.2\cdot 10^{-3} W}{3.6\cdot 10^{-19}J}=3.3\cdot 10^{15}[/tex]
