First of all, we can calculate the frequency of the photon, which is related to the wavelength by the following equation:
[tex]f= \frac{c}{\lambda} [/tex]
where c is the speed of light and [tex]\lambda=670.8 nm=670.8 \cdot 10^{-9} m[/tex] is the photon wavelength. Substituting into the formula, we find the frequency
[tex]f= \frac{c}{\lambda}= \frac{3 \cdot 10^8 m/s}{670.8 \cdot 10^{-9} m}=4.47 \cdot 10^{14} Hz [/tex]
Now we can find the energy of the photon with the following equation:
[tex]E=hf[/tex]
where h is the Planck constant. Substituting numbers, we find
[tex]E=hf=(6.6 \cdot 10^{-34}Js)(4.47 \cdot 10^{14} Hz)=2.95 \cdot 10^{-19} J[/tex]
