where,
ϕ is the work function of the material and
Ek is the kinetic energy of the photo electron.
Here, the wavelength of light in an experiment on photoelectric effect is doubled, that means the frequency of the light is become half. If the energy of the incident light is still grater than or equal to the work function of the material then the electron will emit otherwise not. And in this case stopping potential will decrease.
And as a result, the photoelectric emission will take place