The figure below shows a vaccum tube containing electrodes made of different metals, 1 and 2 whose work functions are `phi_(1)` and `phi_(2)` The electrodes are illuminated simultaneously. The maximum kinetic energy of photoelectrons reaching plate 2 is 1 eV and maximum kinetic energy of photoelectrons reaching plate 1 is 3 eV. Assume that photoelectron emitted from either plate do not interact with each other `phi_(1)=1.5` eV and `phi_(2)=0.7` eV. Find wavelength (in nm) of the electromagnetic wave used

Answer

`400`

Explanation

`K_(2max)=(hc)/lambda-phi_(1)-ev_(0)=1`
`K_(1max)=(hc)/lambdaphi_(2)+ev_(0)=3`
`(2hc)/lambda(phi_(1)+phi_(2))=4`
`(2xx1240)/(4+2.2)=lambda`
`lambda=400nm`