When a metallic surface is illuminated with radiation
of wavelength ``\lambda``, the stopping potential is V. If the
same surface is illuminated with radiation of
wavelength 2``\lambda``, the stopping potential is V/4. The
threshold wavelength for the metallic surface is :-
(1) 4``\lambda``
(2) 5``\lambda``
(3) 5/2 ``\lambda``
(4) 3 ``\lambda``