Question

The stopping potential for photoelectrons emitted from a surface illuminated by light of wavelength 400 mm is 500 mV. When the incident wavelength is changed to a new value, the stopping potential is found to be 800 mV. New wavelength is about-
A. 365m
B. 250 nm
C. 640 nm
D. 340 nm

Answer 1

Correct Answer - A
Energy of photon (1) `E_(1)=(12400)/(lambda(Å)) eV`
I case
`E_(1)=(12400)/(4000)=3.1eV`
`:. V_("stopping")=500mrArrKE ` of emitted `bare=0.5`eV
`:. E_("photon")=W+KEbare`
`rArr` Work function w=2.6 eV
II case
`V_("stopping")=800mvrArrKE_(e^(-))=0.5eV`
`:. E_("photon")=W+KE_(e^(-))`
=2.6+0.8
=3.4 eV
`:. E_("photon")(12400)/(lambda(Å))eV`
` " " 3.4=(12400)/(lambda)`
`" " lambda=3650Å=365nm`