Answer
$\lambda = 497~nm$
Work Step by Step
We can write an expression for the energy of each photon:
$E = \frac{h~c}{\lambda}$
We can write an expression for the photon emission rate:
$R = \frac{P}{E}$
$R = \frac{P~\lambda}{h~c}$
We can find the required wavelength of light:
$R = \frac{P~\lambda}{h~c}$
$\lambda = \frac{R~h~c}{P}$
$\lambda = \frac{(2.50\times 10^{17})(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{0.100~W}$
$\lambda = 4.97\times 10^{-7}~m$
$\lambda = 497~nm$
