Cut-off potentials for a metal in photoelectric effect for light of wavelength ${\lambda }_1,{\lambda }_2$  and   ${\lambda }_3$ is found to be $V_1,V_2$  and  $V_3$volts. If  $V_1,V_2$ and $V_3$  are in arithmetic progression, then  ${\lambda }_1,{\lambda }_2$  and ${\lambda }_3$  will be in