When a certain metallic surface is illuminated with monochromatic light of wavelength `lamda`, the stopping potential for photoelectric current is `3V_0` and when the same surface is illuminated with light of wavelength `2lamda`, the stopping potential is `V_0`. The threshold wavelength of this surface for photoelectrice effect is

Explanation

`3eV_(0)=(hc)/lambda-phi_(0)` and `eV_(0)=(hc)/(2lambda)-phi_(0)`
or `2eV_(0)=(hc)/lambda[1-1/2]=(hc)/(2lambda)` or `(hc)/(eV_(0))=4lambda`
If `lambda_(0)` is the stopping wavelength, then
`eV_(0)=hc//lambda_(0)` or `lambda_(0)=(hc)/(eV_(0))=4lambda`