In the experiment to trace the path of a ray of light through a rectangular glass slab using pins P₁, P₂, P₃ and P₄, four students did the following. The correct procedure is that of the student is: 

The correct procedure to trace the path of a ray of light through a rectangular glass slab using four pins is to look at feet of P₁ and P₂ while placing P₃, and the feet of P₁, P_2, and P₃ while placing P₄.
While performing this experiment, the pins P₁ and P₂ should not be placed too close to each other. Otherwise, it would be really difficult to align the light passing through them when seen from another side of the slab. From the other side, we need to focus on the feet of the pins and then give points that are in line with those two pins.