The energy of a photon with wavelength $\lambda$ is
$\begin{align*}E = h c / \lambda\end{align*}$
where $c$ is the speed of light and $h$ is Planck's constant. If we assume the energy released in the femtosecond pulse is equal to the peak power multiplied by one femtosecond, then the number of photons released is
$\begin{align*}\frac{10 \cdot 10^3 \mbox{ W} \cdot 10^{-15} \mbox{ s}}{h c/\lambda} = \frac{10 \cdot 10^3 \mbox{ W} \cdot 10^{...$
Therefore, answer (B) is the correct answer.

Solution to 1996 Problem 59