= Measuring refraction with a rectangular block
Use a slit <light> source, a rectangular transparent block on plain paper, pencil, ruler and protractor. Trace the block outline, keep it fixed, and mark widely separated points on incident and emergent <light rays>. Remove the block and join the entry and exit points to reconstruct its internal <light ray>. Draw the surface <normal vector> at entry; measure $i$ outside and $r$ inside from that normal line. Choose a path exiting the opposite parallel face rather than a side edge. Repeat for a range of $i$ with the same block and source colour; record $i,r,\sin i,\sin r$ and repeat-reading spread. Plot $\sin i$ vertically against $\sin r$ horizontally: <Snell's law> predicts slope $n_{\mathrm{block}}/n_{\mathrm{air}}$, approximately the block's <refractive index>. The emergent <light ray> should be parallel to the incident one. Beam width, pencil marks, protractor resolution, block movement and colour spread limit precision; a narrow-colour source reduces spread from <optical dispersion>. These are measured angular readings, not a direct timing of <phase velocity>.<International GCSE (Modular) Core Practical Guide>[International GCSE (Modular) Core Practical Guide, Issue 1, Physics core practicals 8–9, printed pp. 171–172 and 178–179: slit source, internal angle convention, pencil crosses, side-edge avoidance and sine-plot axes]{ref}
A <normal incidence> trial checks the drawn normal and the absence of bending, but the ratio $\sin i/\sin r$ is then $0/0$. Small nonzero <angles> also make that ratio sensitive to reading errors. Compare several trials through the slope rather than relying on one nearly axial pair. This rectangular-block measurement complements the existing <total internal reflection> experiment, which probes a different boundary condition.
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