A slit of width 'a' is illuminated by white light. For what value of 'a' will the first minimum for red light of wavelength $\lambda =650 \;nm$ appear for $\theta =30^{\circ}$?

Question

$(a)\;6.5 \times 10^{-7}\;m \\ (b)\;1.3 \times 10^{-7}\;m \\ (c)\;6.5 \times 10^{-6}\;m \\ (d)\;1.3 \times 10^{-6}\;m$