Calculate the mass of a photon with wavelength $3.6 \, \mathring{A}$.

Given wavelength $\lambda = 3.6 \, \mathring{A} = 3.6 \times 10^{-10} \, \text{m}$.
Velocity of photon $v$ is equal to the speed of light $c = 3 \times 10^8 \, \text{m s}^{-1}$.
Using the de Broglie relation $m = \frac{h}{\lambda v}$,where $h = 6.626 \times 10^{-34} \, \text{J s}$:
$m = \frac{6.626 \times 10^{-34} \, \text{J s}}{(3.6 \times 10^{-10} \, \text{m})(3 \times 10^8 \, \text{m s}^{-1})}$
$m = \frac{6.626 \times 10^{-34}}{10.8 \times 10^{-2}} \, \text{kg}$
$m = 6.135 \times 10^{-33} \, \text{kg}$.