If ${ }_{\mathrm{i}} \mu_{\mathrm{j}}$ represents refractive index when ray goes from medium $\mathrm{i}$ to…
If ${ }_{\mathrm{i}} \mu_{\mathrm{j}}$ represents refractive index when ray goes from medium $\mathrm{i}$ to medium $\mathrm{j}$, then the product ${ }_2 \mu_1 \times\mu_2 \times \mu_3$ is equal to