Answer
(a) $1.74\times 10^{18}~photons$
(b) $R = 1.74\times 10^{26}~photons/s$
Work Step by Step
(a) We can find the energy of each photon:
$E = \frac{h~c}{\lambda}$
$E = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{690\times 10^{-9}~m}$
$E = 2.88\times 10^{-19}~J$
We can find the number of photons emitted in each pulse:
$\frac{0.500~J}{2.88\times 10^{-19}~J} = 1.74\times 10^{18}~photons$
(b) We can find the rate of photon emission:
$R = \frac{1.74\times 10^{18}~photons}{10\times 10^{-9}~s}$
$R = 1.74\times 10^{26}~photons/s$
