If the figure below, `PQRS` denotes the path followed by a ray of light as it travels three

If the figure below, `PQRS` denotes the path followed by a ray of light as it travels three media in succession. The absoulte refractive indices of the media are `mu_(1),mu_(2)` and `mu_(3)` respectively (The line segment `RS\'` in the figure is parallel to `PQ`) Then : .