Answer
Please see the work below.
Work Step by Step
We know that
$\nu=\frac{c}{\lambda}$
We plug in the known values to obtain:
$\nu=\frac{2.998\times 10^8}{2,80\times 10^{-6}}=1.0707\times 10^{14} s^{-1}$
We also know that
$E=h\nu$
We plug in the known values to obtain:
$E=6.626\times 10^{-34}\times1.0707\times 10^{14}=7.0945\times 10^{-20}J$
Now $n=4.184\times 10^4\times \frac{1 \space photon}{7.0945\times 10^{-20}}=5.8974\times 10^{23} photons$
