A certain metallic surface is illuminated by monochromatic radiation of wavelength $\lambda$. The stopping potential for photoelectric current for this radiation is $3 \mathrm{~V}_0$. If the same surface is illuminated with a radiation of wavelength $2 \lambda$, the stopping potential is $V_0$. The threshold wavelength of this surface for photoelectric effect is $\_\_\_\_$ $\lambda$.