${\lambda }_1$ and ${\lambda }_2$ are used to illuminate the slits. ${\beta }_1$ and ${\beta }_2$ are the corresponding fringe widths. The wavelength ${\lambda }_1$ can produce photoelectric effect when incident on a metal. But the wavelength ${\lambda }_2$ cannot produce photoelectric effect. The correct relation between ${\beta }_1$ and ${\beta }_2$ is