When a certain photosensistive surface is illuminated with monochromatic light of frequency $v$, the stopping potential for the photo current is $-\frac{\mathrm{V}_0}{2}$. When the surface is illuminated by monochromatic light of frequency $\frac{v}{2}$, the stopping potential is $-\mathrm{V}_0$. The threshold frequency for photoeletric emission is:
