A certain metallic surface is illuminated by monochromatic radiation of wavelength $\lambda .$  The stopping potential for photoelectric current for this radiation is 3V₀. If the same surface is illuminated with a radiation of wavelength $2\lambda ,$  the stopping potential is V₀. The threshold wavelength of this surface for photoelectric effect is_________   $\lambda .$

A beam of electromagnetic radiation of intensity $6.4\times {10}^{-5}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ <!-- [if gte msEquation 12]>6.4×10-5 W/cm2 $\lambda =310\mathrm{n}\mathrm{m}$ <!-- [if gte msEquation 12]>λ=310 nm $\phi =2\mathrm{e}\mathrm{V}$ <!-- [if gte msEquation 12]>φ=2eV $1{\mathrm{c}\mathrm{m}}^2$ <!-- [if gte msEquation 12]>1 cm2 ${10}^3$ <!-- [if gte msEquation 12]>103 ${10}^x.\left(h_c=1240\mathrm{e}\mathrm{V}\mathrm{n}\mathrm{m},1\mathrm{e}\mathrm{V}=1.6\times {10}^{-19}\mathrm{J}\right)$ <!-- [if gte msEquation 12]>10x.hc=1240eVnm,1eV=1.6×10-19 J $\mathrm{x}$ <!-- [if gte msEquation 12]>x