The student draws an incident ray at an angle of incidence $i=35^{\circ }$ to the normal. They place two pins, $P_1$ and $P_2$, on the incident ray to define its path. Suggest a suitable minimum distance apart for pins $P_1$ and $P_2$, and explain why placing them too close together would lead to inaccuracies in the ray trace.

The student views the images of $P_1$ and $P_2$ through the opposite side of the block. They align two more pins, $P_3$ and $P_4$, with these images. (i) State one precaution, other than pin spacing, that the student should take to ensure an accurate alignment of the pins. (ii) State which part of the pins (tops, middles, or bases) the student should look at when aligning them, and give a reason for your choice.

The student plans to test the suggestion that a certain ratio of measured lengths, representing the refraction index relationships, is constant for all possible values of the angle of incidence $i$. List a suitable set of four additional angles of incidence $i$ that the student could use, and state why this selection of angles is appropriate.