The formula for the energy #E# of a photon with wavelength λ is given by the formula
#color(blue)(bar(ul(|color(white)(a/a)E = (hc)/λcolor(white)(a/a)|)))" "#
where
#h =# Planck's constant and
#c =# the speed of light
Thus, the energy of one photon is
#E = (6.626 × 10^"-34"color(white)(l) "J"·color(red)(cancel(color(black)("s"))) × 2.998 ×10^8 color(red)(cancel(color(black)("m·s"^"-1"))))/((2.57 × 10^"-5") × 10^"-9" color(red)(cancel(color(black)("m")))) = 7.729 × 10^"-12" color(white)(l)"J"#
For 1 mol of photons, the energy is
#(7.729 × 10^"-12" color(white)(l)"J")/(1 color(red)(cancel(color(black)("photon")))) × (6.022 × 10^23 color(red)(cancel(color(black)("photons"))))/"1 mol" = 4.65 × 10^12 color(red)(cancel(color(black)("J")))·"mol"^"-1" × "1 kJ"/(1000 color(red)(cancel(color(black)("J")))) = 4.65 × 10^9color(white)(l) "kJ·mol"^"-1"#
