The energy of a photon is 6.69 10-20 J. What is its wavelength (in nanometers)?

Answer 1

A - Convert the energy of the photon into eV dividing by the electron charge (expressed in J): 6.69*10^-20 / 1.6*10^-19 = 0.42 eV.

B - Knowing that the energy E (expressed in eV) is:

E = h*c / Lambda (wavelength in nanometers)

where h is Planck's constant and c is the speed of light (in nm...!), then:

0.42 = 1240 / Lambda

and so:

Lambda = 1240/0.42 = 2965 nm = 2.965 microns

Answer 2

Use the equation E = hc/lambda.. h is Planck's constant, c is the speed of light in meters/sec. When you solve for the wavelength, it will be in meters. Just convert that to nanometers.