We're asked to calculate the energy of one photon of a green light, given its wavelength of
To do this, we can use the equation
$ul(E = (hc)/f$
where
We need to convert from ångströms to meters, using the conversion factor
$1$ $"m"$ $= 10^10$ $"Å"$ :
$5200cancel("Å")((1color(white)(l)"m")/(10^10cancel("Å"))) = color(red)(ul(5.2xx10^-7color(white)(l)"m"$
The energy of the photon is thus
$color(blue)(E) = ((6.626xx10^-34color(white)(l)"J"•cancel("s"))(299792458cancel("m/s")))/(color(red)(5.2xx10^-7cancel("m"))) = color(blue)(ulbar(|stackrel(" ")(" "3.8xx10^-19color(white)(l)"J"" ")|)$ rounded to
$2$ .