The images don't work so I'll write it
When blue light of wavelength 447 bm is shone onto a sodium surface , photoelectrons are emitted . The work function of sodium is 3.28eV
The tresholf frequency is 5.5 *10^14 I worked it out
The energy of photons of the blue light 2.78eV worked it out
Now c is hard for me .
The potential diff that would have to be applied to just prevent photoelemission would be
I don't know where to start
Help