Angular width of the central maxima in the Fraunhofer diffraction for $\lambda=6000 \AA$ is $\theta_{0^{+}}$ When the same slit is illuminated by another monochromatic light, the angular width decreases by $30 \%$. The wavelength of this light is (1) $420 \mathrm{~A}$ (2) $1800 \mathrm{~A}$ (3) $4200 \mathrm{~A}$ (4) $6000 \AA$