A light wave has wavelength $6000 \mathring{A}$ in vacuum. The wavelength of this light when it passes through a certain glass of refraction index $(\mu =1.5 ) $ is

Question

$(a)\;4000 A^{\circ} \\ (b)\;6000 A^{\circ} \\ (c)\;9000 A^{\circ} \\ (d)\;15000 A ^{\circ} $