Question

What is the longest wavelength of light that will provide photons of sufficient energy to break the pi bond and cause the isomerization?

Energy required to break pi bond is 4.42*10^-19.

wavelenght=c/f but how do i figure out what f is?

Answer 1

E = hc/wavelength.
The energy you plug in should be in J/photon (that will be J/mol divided by 6.02 x 10^23 photons/mol).

Answer 2

450 nm