When a piece of metal is illuminated by monochromatic light of wavelength '$\lambda$', the stopping potential for photoelectric current is $2.5 V_0$. When the same surface is illuminated by light of wavelength $1.5 \lambda$, the stopping potential becomes '$V_0$'. The value of threshold wavelength for photoelectric emission is:

$2.25 \lambda$

The correct answer is Option (4) → $2.25 \lambda$